Label-free monitoring of excitation-contraction coupling and excitable cells using impedance based systems with millisecond time resolution

ABSTRACT

A system for monitoring impedance of excitable cells in vitro, which includes a device for monitoring cell-substrate impedance at 20 millisecond resolution, which includes a nonconductive substrate with one or more electrode arrays fabricated in one or more wells, wherein cell attachment on the substrate can result in a detectable change in impedance between electrodes within each electrode array; an impedance analyzer capable of impedance measurement at 20 millisecond time resolution; electronic circuitry that can engage said device and selectively connect said two or more electrode arrays of said device to said impedance analyzer; and a software program that controls said electronic circuitry and records and analyzes data obtained from said impedance analyzer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/435,569, filed May 5, 2009, which claims benefit of priority to U.S.provisional patent application Ser. No. 61/191,684, filed on Sep. 11,2008, and U.S. provisional patent application Ser. No. 61/126,533, filedon May 5, 2008, the content of each is herein incorporated by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates generally to impedance based monitoring ofcells and more particularly to systems and methods for monitoringexcitation-contraction coupling and excitable cells using impedancebased systems with millisecond time resolution.

BACKGROUND OF THE INVENTION

Excitation-contraction coupling (ECC) is a term used to describe thephysiological process of converting an electrical stimulus to amechanical response. The process is fundamental to muscle physiology,wherein the electrical stimulus may be an action potential and themechanical response is contraction. Although ECC has been knowngenerally over half a century, it is still an active area of biomedicalresearch.

Cardiomyocytes are specialized muscle cells of the myocardium that arecapable of excitation-contraction coupling. Cardiomyocytes are commonlyused in biomedical research to assess the cardiotoxicity of potentialdrugs or treatments. Two conventional approaches to assesscardiotoxicity are primarily used. A first approach involves isolationof cardiomyocytes directly from a mammalian species such as rats anddogs followed by electrophysiological studies on the isolatedcardiomyocytes. However, this approach suffers from being extremelylabor-intensive, time consuming and costly and at the same time not veryamenable to the high throughput demands of pharmaceutical industry. Analternative approach utilizes cell-based assay models, whichheterologously express specific ion channels such as hERG channels orvoltage-gated calcium channels. These cardiac ion channels have beenenvisioned as possible molecular targets through which drugs couldinduce cytotoxicity. These cell-based systems allow assessment ofdrug-channel interaction by monitoring the effect of the drug oncurrents produced by different channels in cultured cells using atechnique known as “patch clamping.” Patch clamping isolates regions ofthe cell membrane containing channel proteins and measures changes inelectrical potential difference. However, use of this method in highthroughput requires automation of patch clamping in an array format,which even though is becoming increasing available, is not yetwidespread. In addition, cardiac toxicity may occur by other mechanismsthat may be missed by this type of targeted approach.

An alternative to in vitro ion-channel recording assays as well as thelabor-intensive isolation of primary tissue is the differentiation ofembryonic stem (ES) cells into cardiomyocytes. The utility of ES cellsas a treatment for various chronic diseases has received much attentionin recent years. Mammalian ES cells are self renewing cells derived fromthe inner cell mass of a blastocyst stage embryo which can bedifferentiated into multiple different cell types. It has beendemonstrated that the mouse ES cells as well as human ES cells can bedifferentiated into cardiomyocytes which retain the ability to beat inculture. Differentiation of ES cells first involves an intermediate invitro developmental stage in which ES cells form compact cell structuresknown as embryoid bodies. These embryoid bodies can induce thedevelopmental program of ES cell differentiation into multiple celltypes including cardiomyocytes, which are distinguished in culture bytheir ability to undergo spontaneous beating. These ES derived in vitrodifferentiated cardiomyocytes recapitulates the normal development ofcardiomyocytes as evidenced by the stage-specific expression ofcardiomyocyte specific genes. All the known transcription factors, ionchannels and structural proteins that are part of normal heartdevelopment and function in vivo are also expressed in ES-derivedcardiomyocytes.

Even though high throughput to medium throughput systems have beendeveloped for functional characterization of cell lines heterologouslyexpressing the gene for specific ion channels, high throughputtechniques for functional characterization of more complex systems suchas cardiomyocytes have been limited. Technologies designed to assesscardiomyocyte behaviour and function and the effect of drugs and othermanipulations in vitro can be divided into two different approaches. Oneapproach involves long term assessment of cardiomyocyte viability forexample in response to certain compounds. Such assays are typically endpoint assays designed to measure a cellular component such as ATP whichcorrelates with the degree of viability of the cells. The other approachinvolves studying short term effect of drugs and compounds on beatingfunction of cardiomyocytes. High throughput techniques for short termfunctional characterization of ion channels and other targets incardiomyocytes has been rather challenging and limited. The availablesystems typically only monitor a single cardiomyocyte or a small numberof cardiomyocytes at a time and with very limited throughput.

SUMMARY OF THE INVENTION

The present invention addresses the need to provide systems and methodsfor improved monitoring of excitation-contraction coupling and excitablecells and provides related benefits. Specifically, embodiments of theinvention describe label-free methods for monitoring excitable cells,such as cardiomyocytes, in vitro. The system is capable of continuouslymonitoring excitation-contraction coupling in a relativelyhigh-throughput manner. The systems and methods can be used forpharmacological safety assessment, screening for novel compounds whichmodulate cardiomyocyte function in a specific manner, and assessment ofgenes involved or potentially involved in cardiac function. In additionthe system and methods may study the role of different genes andproteins in cardiac function and development through the use ofembryonic stem (ES) derived cardiomyocytes.

In one aspect of the presence invention a system for monitoringimpedance of excitable cells in vitro is provided including a device formonitoring cell-substrate impedance, an impedance analyzer capable ofimpedance measurements at millisecond time resolution, electroniccircuitry that can engage the device and selectively connect two or moreelectrode arrays of the device to the impedance analyzer and a softwareprogram that controls the electronic circuitry and records and analyzesdata obtained from the impedance analyzer. Embodiments of the device formonitoring substrate impedance include a nonconductive substrate havingone or more individually addressable electrode arrays fabricated thereonand one or more wells. Preferably, each of the one or more arrays isassociated with one of the one or more wells and each electrode array isindividually addressable. In further embodiments, a surface of thesubstrate is suitable for cell attachment, wherein cell attachmentresults in a detectable change in impedance between electrodes withinthe array. Attachment or growth along the surface may be enhanced byapplying a precoat having one or more compounds that improve attachment.In embodiments where the device includes multiple wells, the impedanceanalyzer has the capability to allow impedance measurements for each andevery well at millisecond time resolution. That is to say, for aselected time period, the impedance data for each and every well of thesystem is measured at millisecond time resolution. Further, when groupsof wells are monitored, the groups are monitored at millisecond timeresolution.

Embodiments of the system include electrode arrays having two electrodestructures that have substantially the same surface area. The electrodesthemselves may be formed from suitable electrically conductive materialssuch as metal, gold, platinum, titanium, chromium and the like. Theelectrode arrays may be organized in a variety of configurations,including interdigitated, circle-on-line, diamond-on-line, concentric,sinusoidal, and castellated. The widths of the electrode structures maybe between 20 microns and 500 microns or 5 microns and 100 microns.

Embodiments of the invention include impedance monitoring usingmillisecond time resolution. Millisecond time resolution allowsmonitoring of at least two consecutive impedance measurements with anexcitation cycle of excitable cells, which are introduced into the well.Further, millisecond time resolution enhances measurement such thatexcitation-contraction coupling of excitable cells may be effectivelymonitored. In some embodiments the excitation cycle is a beating cycle,such as a beating cycle of a cardiomyocyte. In some embodiments at leasttwo consecutive impedance measurements are performed in less than 100milliseconds. In other embodiments at least two consecutive impedancemeasurements are performed in less than 40 milliseconds. In stillfurther embodiments at least two consecutive impedance measurements areperformed in less than 20 milliseconds. In still further embodiments theat least two consecutive impedance measurements are performed in less 10milliseconds.

In some embodiments the device is provided as a multiwell device or amultiwell plate. In some embodiments, the multiwell device or multiwellplate may including 6 wells, 8 wells, 16 wells, 32 wells, 96 wells, 384wells or any number therebetween. Throughout the present applicationwhere the device includes multiple wells each of which is associatedwith an electrode array, millisecond time resolution refers to that forany selected time period, the impedance measurement for each and everywell can be performed with two consecutive measurements separated withmillisecond resolution. Further, when groups of wells, such as subsetsof all wells of a device, are monitored a group of wells may bemonitored using millisecond time resolution.

Embodiments of the present invention provide impedance-based monitoringwith millisecond time resolution in combination with longer termmonitoring of cells. By combining millisecond time resolution technologywith impedance monitoring having longer time periods, both short termand long term effects on excitable cells can be evaluated. In someembodiments impedance readouts can be used to monitor both short termbeating and long term viability status of cardiomyocytes in the samewell or the same population. Such monitoring addresses needs wherecertain manipulations such as drug treatment may not manifest its effecton cardiomyocyte beating and/or viability until a later time period.

In some embodiments methods for assessing and quantifying excitablecells in vitro include providing the impedance-based system withmillisecond time resolution; adding excitable cells to one or morewells; monitoring impedance of the one or more wells over a first timeperiod, which is characterized as a longer time period, such as seconds,minutes, hours or days; monitoring impedance of the one or more wellsover a second time period in millisecond time resolution; determining acharacteristic, such as cell attachment, cell growth and cell viabilityfrom monitoring over the first time period; and resolving individualcycles of the excitable cells from monitoring over the second timeperiod. In such embodiments, monitoring impedance over the second timeperiod may be performed at regular or irregular time intervals withinmonitoring impedance over the first time period.

Among the benefits, embodiments of the invention permit impedancereadouts to monitor the morphological or differentiative behavior ofcardiomyocytes in vitro. Certain treatments can induce changes inmorphological behavior of cardiomyocytes, such as inducing hypertrophywhich is associated with cardiomyocyte elongation and expansion. Becauseimpedance monitoring can detect changes in cell morphology, it can beused to for detection of hypertrophy in cardiomyocytes.

In another aspect of the present invention a method for assessing orquantifying excitable cells in vitro is provided. An exemplary methodincludes providing the impedance based system having millisecond timeresolution, adding excitable cells to the one or more wells, monitoringimpedance of the one or more wells, and resolving individual cycles ofthe excitable cells. The method has particular utility with cells thatare known or suspected to be capable of or undergoexcitation-contraction coupling, such as cardiomyocytes. The method mayalso be useful for monitoring neurological cells or neurologicalsystems. Impedance measurements may be performed in millisecond timeresolution. In some embodiments, two consecutive impedance measurementsfor each and every well of the system are performed less than 300milliseconds apart. In further embodiments, at least two consecutiveimpedance measurements for each and every well of the system areperformed less than 100 milliseconds apart. In further embodiments, atleast two consecutive impedance measurements for each and every well ofthe system are performed less than 40 milliseconds apart. In furtherembodiments, at least two consecutive impedance measurements for eachand every well of the system are less than 10 milliseconds apart.

In some embodiments methods of determining whether a compound modulatesa beating cycle of an excitable cell is provided. An exemplary methodincludes providing the impedance based system having millisecond timeresolution, wherein the system includes at least two wells; addingexcitable cells to the at least two wells; monitoring impedance of theat least two wells; adding a compound suspected of modulation of thebeating cycle to a first of the at least two wells to provide a testwell, wherein a second well lacking compound or having a controlcompound is provided as a control well; resolving the beating cycle ofboth test well and control well; and comparing the beating cyclesbetween the test well and control well, wherein a difference in beatingcycles indicates the compound modulates the beating cycle of theexcitable cell. In some embodiments, beating cycles are compared toidentify changes in beating magnitude or amplitude. In otherembodiments, beating cycles are compared to identify changes in beatingfrequency, which may be regular changes or irregular changes. In furtherembodiments the method includes determining a cell index for each of thetest well and control well such that the step of comparing the beatingcycles includes comparing the cell indices between test well and controlwell. Peak magnitude or amplitude and/or frequency may be compared. Themethod may also include monitoring impedance over longer time periods oftime. Such embodiments may provide long term impedance monitoring withintermittent monitoring using millisecond resolution. Thus short termand long term monitoring may be performed.

In another exemplary methods include providing a device for measuringcell-substrate impedance operably connected to an impedance analyzer,wherein the device includes at least two wells; adding ES cells, adultstem-cell derived cardiomyocytes or primary cardiomyocytes to at leasttwo wells; monitoring impedance of the at least two wells at differentor similar time intervals over a period of time and optionallydetermining cell indices from impedance values; generating animpedance-based curve or optionally a cell index curve for each of theat least one known biologically active agent and the control; comparingthe impedance-based curves or optionally the cell index curves betweenthe at least one known biologically active agent well and the controlwell. The impedance-based curves may be direct measurement ofcardiomyocyte excitation-contraction coupling and if significantlydifferent, concluding that the biologically active agent modulatescardiomyocyte function. Optionally, impedance-based curves or optionallycell index curves are used to calculate average rate of beats ofcardiomyocytes per unit time, average amplitude intensity in a unit timeas well as the average length of time between the beats, comparison ofthese optionally derived parameters is made between the at least oneknown biologically active agent well and the control well, and ifsignificant differences exist, one may optionally conclude that thebiologically active agents modulate cardiomyocyte functions.

In other embodiments of the present invention, a method ofcharacterizing rhythmic beating of a cardiomyocyte is provided. Anexemplary method includes providing the impedance based system withmillisecond time resolution; adding excitable cells to the one or morewells; monitoring impedance of the one or more wells in millisecond timeresolution; determining a plurality of beating cycle peaks; andcomparing magnitude or amplitude or frequency of the peaks over a timeunit. Various methods can be used for determining beating cycle peaks.For example, time dependent impedance values or cell index values for awell are analyzed by deriving their first order derivatives and secondorder derivatives using numerical methods. The beating cycle peaks arethose data points where the first order derivatives of impedance valuesor cell index values are zero or close to zero in its absolute value. Ifthe beating cycle peak is a positive peak (i.e. peak corresponds to amaximum value in measured impedance or cell index over the beatingcycle), then the peak would correspond to the data points where thesecond order derivatives of the impedance values or cell index values isnegative and where the first order derivatives of the impedance valuesor cell index values is zero or close to zero in its absolute value. Ifthe beating cycle peak is a negative peak (i.e. peak corresponds to aminimum value in measured impedance or cell index over the beatingcycle), then the peak would correspond to the data points where thesecond order derivatives of the impedance values or cell index values ispositive and where the first order derivatives of the impedance valuesor cell index values is zero or close to zero in its absolute value. Thestep of determining a plurality of beating cycle peaks may includedetermining a rise in impedance and a decay in impedance andextrapolating a peak from each rise and decay. The method may furtherinclude determining a cell index for each of the one or more wells,which may allow improved comparison of beating cycles. Further, beatingcycle peaks may be determined by identifying frequency components havinglargest magnitude in Fourier transformed impedance data. The method mayalso include monitoring impedance over longer time periods of time. Suchembodiments may provide long term impedance monitoring with intermittentmonitoring using millisecond resolution.

A variety of agents and compounds may be tested for modulation ofexcitable cells and for modulation of rhythmic beating ofcardiomyocytes. Examples include compounds, peptides, proteins,antibodies or antibody fragments, siRNA, shRNA, lipid or any combinationof thereof.

In another aspect of the present invention a method of assessing geneticmanipulation of an embryonic stem cell is provided including providingthe impedance based system having millisecond time resolution, mutatingembryonic stem cells to include a genetic mutation; adding mutatedembryonic stem cells to a first well of at least two wells to form atest well and control embryonic stem cells to a second of at least twowells to form a control well; monitoring impedance of the at least twowells in millisecond time resolution; resolving the beating cycles ofboth test well and control well, if any; and comparing the beatingcycles between the test well and control well, wherein a difference inbeating cycles indicates the genetic mutation modulates embryonic stemcell differentiation. In some embodiments the embryonic stem celldifferentiation is differentiation into a cardiomyocyte. In someembodiments the genetic mutation is selected from the group consistingof a genetic knock out, a knock down and an inserted transgene. Themethod may also include monitoring impedance over longer time periods oftime. Such embodiments may provide long term impedance monitoring withintermittent monitoring using millisecond resolution. Thus short termand long term monitoring may be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art will recognize that the drawings describedbelow are for illustrative purposes only. The drawings are not intendedto limit the scope of the invention but to provide exemplaryembodiments.

FIGS. 1A-1B depict schematic drawings of one design of a cell-substrateimpedance measurement device of the present invention. FIG. 1A shows anonconductive substrate (101) shown with 16 electrode arrays fabricatedon the substrate. Each electrode array (102) comprises two electrodestructures (shown in detail in FIG. 1B). Each electrode structurecomprises multiple electrode elements. Each electrode array connects totwo electrical traces (103), with each of the two traces connected oneof the two electrode structures. These electrical connection traces(103) from the electrode array (102) are connected to the connectionpads (104) at the edges of the substrate (101). As shown in FIG. 1A,each the four electrode arrays in each of four quarters on the substrate(101) have one of their electrical connection traces (103) connected toa common connection pad (104). Thus, for the entire device there arefour common connection pads (104), one for each quarter of the device.In addition, each electrode array has a separate electrical connectiontrace (103), connecting to an independent connection pad (104). Thus,there are total 20 connection pads (104) at the edges of the substrate(101). In FIG. 1B a single exemplary electrode array is shown. Theelectrode array has two electrode structures, where each electrodestructure comprises multiple electrode elements (105) shown here havinga circle-on-line geometry. In this electrode array structure, electrodeelements (105) of one electrode structure of the array alternate withelectrode elements (105) of the other electrode structure of the array.Each of the electrode structures is independently connected to itselectrode bus (106), in this case, by means of direct connection of theelectrode elements (105) to the electrode bus (106). Each electrode bus(106) forms an arc around the perimeter of the array, where the twobuses of the array do not abut or overlap. The electrically conductiveconnection traces (103 in FIG. 1A) connect each bus with a connectionpad (104 in FIG. 1A) on the edge of the substrate (101 in FIG. 1A).

FIG. 2A is a schematic representation of a device 100 with two electrodestructures of same or similar areas deposited on a substrate. Firstelectrode structure has electrode elements 110 a, 110 b, 110 c andsecond electrode structure has electrode elements 120 a, 120 b, 120 cand 120 d. Electrode elements within an electrode structure areconnected to each other by arc-shaped connection electrode bus 125. Likethe electrode elements, such connection-buses (125) are also made ofelectrically-conductive material (e.g. gold film, platinum film, goldfilm over a chromium or titanium film). These electrically-conductiveconnection-paths or connection buses (125) may have an insulatingcoating. Electrode elements 110 a-110 c and 120 a-120 d compriseelectrode lines with connected circles added on the line. The overallarea of electrode elements and gaps between electrode elements maycorrespond to, or may be slightly larger than, or may be slightlysmaller than, the bottom of a well (e.g., a cylinder shaped well, aconical shaped well, or a cubic shaped well), for example, a 24well-plate, a 96-well plate, or 384 well plate that are commonly used.The whole surfaces of the wells may be covered with electrodes to ensurethat the cell attachment at nearly any locations of the bottom surfaceof the well can contribute to the impedance change. This arrangement hasan advantage that non-uniform landing and attachment of cells on thebottom surface of the different wells would result in only a smallvariation in the impedance measured between electrode structures 110 and120. 150 are connection pads that can be connected to an externalimpedance measurement circuit. 130 is the electrical connection tracesthat connects the connection pad to the electrode structures 110 and120. Such connection traces can extend in any direction in the plane ofthe electrodes. FIG. 2B is a schematic representation of a device 200with two electrode structures of similar areas deposited on a substrate.Electrode structures 210 and 220 comprise multiple interconnectedelectrode elements. Electrode elements (210 a-210 c, 220 a-220 d) arerectangular lines and together form an interdigitated electrodestructure unit. Similar to FIG. 2A, the electrode elements (210 a-210 c,220 a-220 d) within each electrode structure are connected througharc-shaped, electrically conductive paths or electrode buses (225).Connection pads 250 are connected to electrode structures through theelectrical connection traces 230. FIG. 2C is a schematic representationof a device 300 with two electrode structures of similar areas depositedon a substrate. Electrode structures 310 and 320 comprise multipleinterconnected electrode elements (310 a-310 f, 320 a-320 f). Electrodeelements (310 a-310 c, 320 a-320 d) are rectangular lines and togetherform an interdigitated electrode structure unit. Different from FIG. 2Aand FIG. 2B, the electrode structures having electrode elements 310a-310 c and 320 a-320 c are connected to connection pads 350.

FIG. 3 shows the cell index curves measured on RT-CES system for 4different seeding densities (3750, 7500, 15,000 and 30,000 cells perwell) of mouse ES-derived cardiomyocytes. In this example, the impedancemeasurement was taken with a time resolution of 1 hour between any twoconsecutive data points for each well.

FIG. 4A provides a plot of normalized cell index values for cells indifferent wells treated with different concentrations of sodiumdichromate dehydrate (SDD). FIG. 4B provides the corresponding sigmoidalcurve for deriving the IC-50 of the compound.

FIG. 5A provides a graph of normalized impedance data over time showingshifts in morphology in response to isproteranol, a β2 adrenergicreceptor agonist. FIG. 5B shows a plot of the normalized cell indicesagainst the log of the corresponding isoproteranol concentrations tocalculate the EC-50.

FIG. 6 shows the cell index curves measured on RT-CES system for 4individual wells, with cell culture medium as background starting fromcell seeding to about 86 hrs after cell seeding. For such long termmeasurement, cell electrode impedance and corresponding cell indiceswere measured at about 15 minute intervals. Thus for each well, theimpedance measurement was performed with a time resolution of 15minutes.

FIGS. 7A-D provides a series of graphs showing cardiomyocyte beatingthroughout attachment and growth phases.

FIG. 8A and FIG. 8B are a pair of cell index curves and thecorresponding beat rate derived from Fourier transformation of impedancevalues

FIGS. 9A-D show cardiomyocyte beating data from mouse cardiomyocytestreated with sotalol. In FIGS. 9C and 9D, the time resolution betweentwo adjacent points is 40 milliseconds. In other words, a second inFIGS. 7C and 7D is equivalent to 40 milliseconds.

FIG. 10A depicts graphs of agents which slow down heart rate andcardiomyocyte beating. FIG. 10B depicts graphs of agents which increaseheart rate and cardiomyocyte beating.

FIGS. 11A-S depict impedance data of cardiomyocyte patterns associatedwith testing of a variety of known compounds, which are furtherdescribed in Table II.

FIG. 12 depicts impedance based cardiotoxic patterns from knowncompounds.

FIG. 13 shows an example of dose dependent effects of Astemizole oncardiomyocytes beating at different concentrations.

FIGS. 14A-H are graphical depictions of real time impedance-basedmillisecond time resolution of cardiomyocyte beating in response toHaloperidol treatment. The Y-axis is the cell index value.

FIGS. 15A-H depict graphical analysis of the impedance beating frequencyfrom impedance plots (cell index plots) shown in FIGS. 14A-H.

FIGS. 16A-H depict additional graphical analysis of the magnitude oramplitude of the beating cycle peaks from impedance plots (cell indexplots) shown in FIGS. 14A-H.

FIGS. 17A-I are graphical depictions of real time impedance-basedmillisecond time resolution of cardiomyocyte beating in response toErythromycin treatment. The Y-axis is the cell index value.

FIGS. 18A-I depict graphical analysis of the impedance beating frequencyfrom impedance plots (cell index plots) shown in FIGS. 17A-I.

FIGS. 19A-I depict additional graphical analysis of beating magnitude oramplitude of the beating cycle peaks shown from impedance plots (cellindex plots) shown in FIGS. 17A-I.

DETAILED DESCRIPTION OF THE INVENTION

Detailed descriptions of preferred embodiments of the provided herein.It is to be understood; however, that the present invention may beembodied in various forms. Therefore specific reference to various formsare provided as a basis for the claims and for teaching one skilled inthe present art to employ the present invention in appropriate system,structure or manner.

A. Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as is commonly understood by one of ordinary skillin the art to which this invention belongs. All patents, applications,published applications and other publications referred to herein areincorporated by reference in their entirety. If a definition set forthin this section is contrary to or otherwise inconsistent with adefinition set forth in the patents, applications, publishedapplications and other publications that are herein incorporated byreference, the definition set forth in this section prevails over thedefinition that is incorporated herein by reference.

As used herein, “a” or “an” means “at least one” or “one or more.”

As used herein, “membrane” is a sheet of material.

As used herein, “biocompatible polymer” means a polymeric material thatdoes not have deleterious effects on cells, including the viability,attachment, spreading, motility, growth, or cell division.

As used herein, “suitable for attachment” refers to structural featuresthat permit attachment of cells thereto. A surface is “suitable for cellattachment” when a significant percentage of the cells attach or adhereto the surface within twelve hours. Preferably, at least 50% of thecells are adhered to the surface within twelve hours. More preferably,at least 70% of the cells are adhered to the surface within twelve hoursof plating (i.e., adding cells to the well or fluid container). Evenmore preferably, the surface properties of a surface that is suitablefor cell attachment results in at least 90% of the cells adhering to thesurface within twelve hours of plating. Most preferably, the surfaceproperties of a surface that is suitable for cell attachment results inat least 90% of the cells adhering to the surface within eight, six,four, two hours of plating. To have desired surface properties for cellattachment, the surface may need to chemically-treated (e.g. treatmentwith an acid and/or with a base), and/or physically treated (e.g.treatment with plasma), and/or biochemically treated (e.g. coated withone or more molecules or biomolecules that promotes cell attachment).

As used herein, “impedance measurement at millisecond time resolution”refers to the ability to perform a series of at least two consecutiveimpedance measurements within milliseconds of one another for each andevery well of the system where each well is associated with an electrodearray for impedance measurement. In each instance the time between twoconsecutive impedance measurements are less than 300 milliseconds fromone another. Preferably, consecutive impedance measurements are lessthan 200 milliseconds from one another. More preferably impedancemeasurements are less than 100 milliseconds from one another. Forexample, the time between two consecutive impedance measurements are 40milliseconds from one another for each and every well of the system. Tobe clear, that is to say, in a time period of 40 milliseconds, thesystem has the ability to measure two impedance data points for each andevery well of the system.

As used herein, “excitable cells” refers to a cell population that iscapable of propagation and spreading action potentials to surroundingcells. These cells are able to produce and respond to electricalsignals. Some excitable cells are capable of excitation-contractioncoupling. Examples of “excitable cells” include cardiomyocytes, cellsthat beat, some skeletal muscle cells, some smooth muscle cells and someneural cells. Identification of excitable cells may be found in cellbiology literature.

As used herein, “excitation-contraction coupling” refers to thephysiological process of converting an electrical stimulus to mechanicalresponse. This process is fundamental to muscle physiology, whereby theelectrical stimulus is usually an action potential and the mechanicalresponse is contraction. EC coupling can be dysregulated in many diseaseconditions.

As used herein, “beating cycle” refers the time between which acardiomyocyte sequentially beats. The “beating cycle” may vary dependingon organism and developmental stage of the cardiomyocyte. A beatingcycle may be as short as between 100 and 200 milliseconds, translatingto a beating rate of 600 to 300 beats per minute. A beating cycle may beas long as between 1000 millisconds and 10,000 milliseconds, translatingto a beating rate of 60 to 6 beats per minute.

As used herein, “biomolecular coating” or “coated with a biomolecule”refers to is a coating on a surface that comprises a molecule that is anaturally occurring biomolecule or biochemical, or a biochemical derivedfrom or based on one or more naturally occurring biomolecules orbiochemicals. For example, a biomolecular coating can comprise anextracellular matrix component (e.g., fibronectin, collagens), or aderivative thereof, or can comprise a biochemical such as polylysine orpolyornithine, which are polymeric molecules based on the naturallyoccurring biochemicals lysine and ornithine. Polymeric molecules basedon naturally occurring biochemicals such as amino acids can use isomersor enantiomers of the naturally-occuring biochemicals.

As used herein, “extracellular matrix component” refers to a moleculethat occurs in the extracellular matrix of an animal. It can be acomponent of an extracellular matrix from any species and from anytissue type. Nonlimiting examples of extracellular matrix componentsinclude laminins, collagens fibronectins, other glycoproteins, peptides,glycosaminoglycans, proteoglycans, etc. Extracellular matrix componentscan also include growth factors.

As used herein, “electrode” refers to a structure having a highelectrical conductivity, that is, an electrical conductivity much higherthan the electrical conductivity of the surrounding materials.

As used herein “electrode structure” refers to a single electrode,particularly one with a complex structure (as, for example, a spiralelectrode structure), or a collection of at least two electrode elementsthat are electrically connected together. All the electrode elementswithin an “electrode structure” are electrically connected.

As used herein, “electrode element” refers to a single structuralfeature of an electrode structure, such as, for example, a fingerlike orbranched projection of an interdigitated electrode structure.

As used herein “electrode structure unit” refers to two or moreelectrode structures that are constructed to have dimensions and spacingsuch that they can, when connected to a signal source, operate as a unitto generate an electrical field in the region of spaces around theelectrode structures. Preferred electrode structure units of the presentinvention can measure impedance changes due to cell attachment to anelectrode surface. Non-limiting examples of electrode structure unitsare interdigitated electrode structure units and concentric electrodestructure units.

As used herein, “electrode traces” refer to electrically conductivepaths that extend from electrodes or electrode elements or electrodestructures toward one end or boundary of a device or apparatus forconnecting the electrodes or electrode elements or electrode structuresto an impedance analyzer. The end or boundary of a device may correspondto the connection pads on the device or apparatus.

As used herein, “connection pad” refers to an area on an apparatus or adevice of the present invention which is electrically connected to atleast one electrode or all electrode elements within at least oneelectrode structure on an apparatus or a device and which can beoperatively connected to external electrical circuits (e.g., animpedance measurement circuit or a signal source). The electricalconnection between a connection pad and an impedance measurement circuitor a signal source can be direct or indirect, through any appropriateelectrical conduction means such as leads or wires. Such electricalconduction means may also go through electrode or electrical conductionpaths located on other regions of the apparatus or device.

As used herein, “interdigitated” refers to having projections comingfrom one direction that interlace with projections coming from adifferent direction in the manner of the fingers of folded hands (withthe caveat that interdigitated electrode elements preferably do notcontact one another).

As used herein, “high probability of contacting an electrode element”refers to if a cell is randomly positioned within the sensor area of adevice or apparatus of the present invention, the probability of a cell(or particle) contacting on an electrode element, calculated from theaverage diameter of a cell used on or in a device or apparatus of thepresent invention, the sizes of the electrode elements, and the size ofthe gaps between electrode elements, is greater than about 50%, morepreferably greater than about 60%, yet more preferably greater thanabout 70%, and even more preferably greater than about 80%, greater thanabout 90%, or greater than about 95%.

As used herein, “at least two electrodes fabricated on said substrate”refers to at least two electrodes are fabricated or made or produced onthe substrate. The at least two electrodes can be on the same side ofthe substrate or on the different side of the substrate. The substratemay have multiple layers, the at least two electrodes can be either onthe same or on the different layers of the substrate.

As used herein, “at least two electrodes fabricated to a same side ofsaid substrate” refers to at least two electrodes are fabricated on thesame side of the substrate.

As used herein, “at least two electrodes fabricated to a same plane ofsaid substrate” refers to, if the nonconducting substrate has multiplelayers, the at least two electrodes are fabricated to the same layer ofthe substrate.

As used herein, “electrodes have substantially same surface area” refersto the surface areas of the electrodes referred to are not substantiallydifferent from each other, so that the impedance change due to cellattachment or growth on any one of the electrodes referred to willcontribute to the overall detectable change in impedance to a same orsimilar degree as the impedance change due to cell attachment or growthon any other of the electrodes referred to. In other words, whereelectrodes have substantially the same surface area, any one of theelectrodes can contribute to overall change in impedance upon cellattachment or growth on the electrode. In most cases, the ratio ofsurface area between the largest electrode and the smallest electrodethat have “substantially the same surface area” is less than 10.Preferably, the ratio of surface area between the largest electrode andthe smallest electrode of an electrode structure is less than 5, 4, 3,2, 1.5, 1.2 or 1.1. More preferably, the at least two electrodes of anelectrode structure have nearly identical or identical surface area.

As used herein, “said device has a surface suitable for cell attachmentor growth” refers to the electrode and/or non-electrode area of theapparatus has appropriate physical, chemical or biological propertiessuch that cells of interest can viably attach on the surface and newcells can continue to attach, while the cell culture grows, on thesurface of the apparatus. However, it is not necessary that the device,or the surface thereof, contain substances necessary for cell viabilityor growth. These necessary substances, e.g., nutrients or growthfactors, can be supplied in a medium. Preferably, when a suspension ofviable, unimpaired, excitable cells is added to the “surface suitablefor cell attachment” at least 50% of the cells adhere to the surfacewithin twelve hours. More preferably, a surface that is suitable forcell attachment has surface properties so that at least 70% of the cellsare adhering to the surface within twelve hours of plating (i.e., addingcells to the chamber or well that comprises the said device). Even morepreferably, the surface properties of a surface that is suitable forcell attachment results in at least 90% of the cells adhering to thesurface within twelve hours of plating. Most preferably, the surfaceproperties of a surface that is suitable for cell attachment results inat least 90% of the cells adhering to the surface within eight, six,four, two hours of plating.

As used herein, “detectable change in impedance between or among theelectrodes” refers to the impedance between or among the electrodeswould have a significant change that can be detected by an impedanceanalyzer or impedance measurement circuit when molecule binding reactionor cell attachment or cell adhesion or cell presence occurs on theelectrode surfaces. The impedance change refers to the difference inimpedance values when cell attachment or cell adhesion or cell presenceor molecule binding reaction occurs on the electrode surface of theapparatus and when no molecular reaction occurs on the electrode surfaceor no cell is present on the electrode surface. The impedance change mayoccur upon the presence or decay of a beat from a cardiomyocyte.Alternatively, the impedance change refers to the difference inimpedance values when cells are attached to the electrode surface andwhen cells are not attached to the electrode surface, or when thenumber, type, activity, or morphology of cells attached to theelectrode-comprising surface of the apparatus changes. In most cases,the change in impedance is larger than 0.1% to be detectable.Preferably, the detectable change in impedance is larger than 1%, 2%,5%, or 8%. More preferably, the detectable change in impedance is largerthan 10%. Impedance between or among electrodes is typically a functionof the frequency of the applied electric field for measurement.“Detectable change in impedance between or among the electrodes” doesnot require the impedance change at all frequencies being detectable.“Detectable change in impedance between or among the electrodes” onlyrequires a detectable change in impedance at any single frequency (ormultiple frequencies). In addition, impedance has two components,resistance and reactance (reactance can be divided into two categories,capacitive reactance and inductive reactance). “Detectable change inimpedance between or among the electrodes” requires only that either oneof resistance and reactance has a detectable change at any singlefrequency or multiple frequencies. In the present application, impedanceis the electrical or electronic impedance. The method for themeasurement of such impedance is achieved by, (1) applying a voltagebetween or among the electrodes at a given frequency (or multiplefrequencies, or having specific voltage waveform) and monitoring theelectrical current through the electrodes at the frequency (or multiplefrequencies, or having specific waveform), dividing the voltageamplitude value by the current amplitude value to derive the impedancevalue; (2) applying an electric current of a single frequency component(or multiple frequencies or having specific current wave form) throughsaid electrodes and monitoring the voltage resulted between or among theelectrodes at the frequency (or multiple frequencies, or having specificwaveform), dividing the voltage amplitude value by the current amplitudevalue to derive the impedance value; (3) other methods that can measureor determine electric impedance. Note that in the description above of“dividing the voltage amplitude value by the current amplitude value toderive the impedance value”, the “division” is done for the values ofcurrent amplitude and voltage amplitude at same frequencies. Measurementof such electric impedance is an electronic or electrical process thatdoes not involve the use of any reagents.

As used herein, “multiple pairs of electrodes or electrode structuresspatially arranged according to wells of a multi-well microplate” refersto multiple pairs of electrodes or electrode structures of a device orapparatus are spatially arranged to match the spatial configuration ofwells of a multi-well microplate so that, when desirable, the device canbe inserted into, joined with, or attached to a multiwell plate (forexample, a bottomless multiwell plate) such that multiple wells of themulti-well microplate will comprise electrodes or electrode structures.

As used herein, “arranged in a row-column configuration” refers to that,in terms of electric connection, the position of an electrode, anelectrode array or a switching circuit is identified by both a rowposition number and a column position number.

As used herein, “each well contains substantially same number . . . ofcells” refers to the lowest number of cells in a well is at least 50% ofthe highest number of cells in a well. Preferably, the lowest number ofcells in a well is at least 60%, 70%, 80%, 90%, 95% or 99% of thehighest number of cells in a well. More preferably, each well containsan identical number of cells.

As used herein, “each well contains . . . same type of cells” meansthat, for the intended purpose, each well contains same type of cells;it is not necessary that each well contains exactly identical type ofcells. For example, if the intended purpose is that each well containsmammalian cells, it is permissible if each well contains same type ofmammalian cells, e.g., human cells, or different mammalian cells, e.g.,human cells as well as other non-human mammalian cells such as mice,goat or monkey cells, etc. In some instances the “same type of cells”may initially refer to embryonic stem cells; however, the stage ofdifferentiation into cardiomyocytes may be differ.

As used herein, “each well contains . . . serially differentconcentration of a test compound” means that each well contains a testcompound with a serially diluted concentrations, e.g., an one-tenthserially diluted concentrations of 1 M, 0.1 M, 0.01 M, etc.

As used herein, “dose-response curve” means the dependent relationshipof response of cells on the dose concentration of a test compound. Theresponse of cells can be measured by many different parameters. Forexample, a test compound is suspected to be cardiotoxic if it impairscardiomyocyte beating, whether magnitude or amplitude or frequency, orif it causes cell death. Then the response of cells can be measured bypercentage of non-viable (or viable) cardiac cells after the cells aretreated by the test compound.

As used herein, “microelectrode strip or electrode strip” refers to anon-conducting substrate strip on which electrodes or electrodestructure units are fabricated or incorporated. The non-limitingexamples of the non-conducting substrate strips include polymermembrane, glass, plastic sheets, ceramics, insulator-on-semiconductor,fiber glass (like those for manufacturing printed-circuits-board).Electrode structure units having different geometries can be fabricatedor made on the substrate strip by any suitable microfabrication,micromachining, or other methods. Non-limiting examples of electrodegeometries include interdigitated electrodes, circle-on-line electrodes,diamond-on-line electrodes, castellated electrodes, or sinusoidalelectrodes. Characteristic dimensions of these electrode geometries mayvary from as small as less than 5 micron, or less than 10 micron, to aslarge as over 200 micron, over 500 micron, over 1 mm. The characteristicdimensions of the electrode geometries refer to the smallest width ofthe electrode elements, or smallest gaps between the adjacent electrodeelements, or size of a repeating feature on the electrode geometries.The microelectrode strip can be of any geometry for the presentinvention. One exemplary geometry for the microelectrode strips isrectangular shape—having the width of the strip between less than 50micron to over 10 mm, and having the length of the strip between lessthan 60 micron to over 15 mm. An exemplary geometry of themicroelectrode strips may have a geometry having a width of 200 micronand a length of 20 mm. A single microelectrode strip may have twoelectrodes serving as a measurement unit, or multiple suchtwo-electrodes serving as multiple measurement units, or a singleelectrode structure unit as a measurement unit, or multiple electrodestructure units serving as multiple electrode structure units. In oneexemplary embodiment, when multiple electrode structure units arefabricated on a single microelectrode strip, these electrode structureunits are positioned along the length direction of the strip. Theelectrode structure units may be of squared-shape, or rectangular-shape,or circle shapes. Each of electrode structure units may occupy size fromless than 50 micron by 50 micron, to larger than 2 mm×2 mm.

A “compound” or “test compound” is any compound whose activity or director indirect effect or effects on cells is investigated in any assay. Atest compound can be any compound, including, but not limited to, asmall molecule, a large molecule, a molecular complex, an organicmolecule, an inorganic molecule, a biomolecule or biological moleculesuch as but not limited to a lipid, a steroid, a carbohydrate, a fattyacid, an amino acid, a peptide, a protein, a nucleic acid, or anycombination thereof. A test compound can be a synthetic compound, anaturally occurring compound, a derivative of a naturally-occurringcompound, etc. The structure of a test compound can be known or unknown.In one application of the present invention, a compound is capable of,or is suspected of, modulating the beating cycle of a cardiomyocyte cellpopulation, whether by beating amplitude or frequency. In still anotherapplication, a compound is capable of, or is suspected of, interactingwith cells (for example, binding to cell surface receptor, or inhibitingcertain intracellular signal transduction pathway, or activating cells).

A “known compound” is a compound for which at least one activity isknown. In the present invention, a known compound preferably is acompound for which one or more direct or indirect effects on cells isknown. Preferably, the structure of a known compound is known, but thisneed not be the case. Preferably, the mechanism of action of a knowncompound on cells is known, for example, the effect or effects of aknown compound on cells can be, as nonlimiting examples, effects oncardiotoxicity or embryonic stem cell development, etc.

An “impedance value” is the impedance measured for electrodes in a wellwith or without cell present. Impedance is generally a function of thefrequency, i.e., impedance values depend on frequencies at which themeasurement was conducted. For the present application, impedance valuerefers to impedance measured at either single frequency or multiplefrequencies. Furthermore, impedance has two components, one resistancecomponent and one reactance component. Impedance value in the presentapplication refers to resistance component, or reactance component, orboth resistance and reactance component. Thus, when “impedance value”was measured or monitored, we are referring to that, resistance, orreactance, or both resistance and reactance were measured or monitored.In many embodiments of the methods of the present application, impedancevalues also refer to parameter values that are derived from raw,measured impedance data. For example, cell index, or normalized cellindex, or delta cell index could be used to represent impedance values.

A “Cell Index” or “CI” is a parameter that can derived from measuredimpedance values and that can be used to reflect the change in impedancevalues. There are a number of methods to derive or calculate Cell Index.

A “Normalized Cell Index” at a given time point is calculated bydividing the Cell Index at the time point by the Cell Index at areference time point. Thus, the Normalized Cell Index is 1 at thereference time point.

A “delta cell index” at a given time point is calculated by subtractingthe cell index at a standard time point from the cell index at the giventime point. Thus, the delta cell index is the absolute change in thecell index from an initial time (the standard time point) to themeasurement time.

A “Cell Change Index” or “CCI” is a parameter derived from Cell Indexand “CCI” at a time point is equal to the 1^(st) order derive of theCell Index with respect to time, divided by the Cell Index at the timepoint. In other words, CCI is calculated as

${{CCI}(t)} = {\frac{{dCI}(t)}{{{CI}(t)} \cdot {dt}}.}$

As used herein, “primary cell” or “primary cells” refers to anynon-immortalized cell that has been derived from various tissues andorgans of a patient or an animal.

B. Introduction to Impedance-Based Systems and Methods HavingMillisecond Time Resolution

The impedance-based system and methods of the present invention fill amajor technological gap in monitoring of excitation-contraction couplingand excitable cells, such as cardiomyocytes, in vitro. This isaccomplished, in part, from the development of an impedance-based systemand monitoring method that employs millisecond time resolution.Millisecond timer resolution permits effective monitoring of excitablecells for short durations, such as less than about 100 milliseconds.Further by coupling millisecond time resolution with longer durationtime periods, both short term and long term effects of drugs, such ascell viability, changes in morphology and adhesion can also be assessed.These combination studies permit further data collection with respect tocompounds such as β-2 adrenergic receptor agonists, well known andcharacterized modulators of heart function in vivo and in vitro, whichcan induce long term hypertrophic responses in cardiomyocytes, which isassociated with elongated morphology of the cells. When combining longterm monitoring and short term monitoring it may be desirable to providelong term monitoring coupled to intermittent short term monitoringperiods.

The devices, systems and methods provided permit impedance-basedmonitoring of excitable cells using millisecond time resolution. It hasbeen discovered that by developing enhanced impedance based systems,detailed analysis of excitable cells, the excitation cycle of cells andpotential modulators of excitable cells can be performed. Further, bystudying the excitation cycle with increased resolution embodiments ofthe invention permit the effective evaluation of factors that may alternormal or abnormal cycling. Without millisecond time resolution, itwould not be possible to perform detailed analysis of the beating cycleof the excitable cells. For example, for cardiomyocytes, if theimpedance is measured at seconds time-resolution such as a ten-second oreven one-second resolution, it would not be possible to observe beatingof the cardiomyocytes or resolve a beating cycle of the cardiomyocyteswith an impedance measurement. The cardiomyocytes-beating-resultedimpedance change would be shown as “noise” on the impedance curvesmeasured at seconds or minutes time-resolution. Indeed, it was asurprise that the beating of the cardiomyoctes on the electrode surfaceswould result in periodic change in impedance and even more surprise thatthe impedance measurement, when performed at appropriate time resolutionof milli-seconds, could resolve such beating cycles. After we discoveredthat cardiomyocyte beating, when cultured on electrode surfaces, can bemonitored via impedance measurement, we interpreted that such dynamicmonitoring of cardiomyocyte beating is based on quantification in realtime of the rhythmic changes in cardiomyocyte morphology and/orattachment as a result of the excitation contraction coupling of theelectrically excitable cardiomyocytes growing on microelectrodes'surface in E-PLATES (ACEA Biosciences, San Diego, Calif.). Thequantification of the rhythmic changes in cardiomyocyte morphologyand/or attachment is achieved via the fast (millisecond resolution) andcontinuous measurement of electrode impedance. The method essentiallyprovides a cellular cardio-gram which can provide incisive informationabout the status of cardiomyocytes especially upon treatment withpharmacological agents.

When combining long term impedance monitoring and short term monitoringit may be desirable to provide long term monitoring coupled tointermittent short term monitoring. In other words, while long termimpedance monitoring is able to identify long term effects on excitablecells, short term impedance monitoring, such as at millisecond timeresolution may be employed at particular time points to assess shortterm effects. In some instances, impedance monitoring with millisecondtime resolution is performed at regular or irregular time intervals. Inthese instances, time intervals may be preprogrammed in the system ormay be initiated by the user. In some instances intermittent measuringat millisecond time resolution is initiated upon the occurrence of anevent, such as an event detected or determined while long termmonitoring of impedance. Such events may be changes in measuredimpedance data, changes in optical property and the like. Monitoring atmillisecond time resolution may continue until a programmed eventoccurs, such as expiration of a time period. After or during which, longterm impedance monitoring may continue.

In some embodiments, long term and short term monitoring ofexcitation-contraction coupling or excitable cells is provided incombination. Exemplary methods include providing the impedance-basedsystem with millisecond time resolution; adding excitable cells or cellssuspected of excitation-contraction coupling to one or more wells;monitoring impedance of the one or more wells over a first time period,which is characterized as a longer time period, such as seconds,minutes, hours or days; monitoring impedance of the one or more wellsover a second time period in millisecond time resolution; determining acharacteristic, such as cell attachment, cell growth and cell viabilityfrom monitoring over the first time period; and resolving individualcycles of the excitable cells from monitoring over the second timeperiod. In such embodiments, monitoring impedance over the second timeperiod may be performed at regular or irregular time intervals withinmonitoring impedance over the first time period. Further, test agentsmay be added to assess their short term effects and long term effects onthe cells.

Impedance monitoring using, at least in part, millisecond timeresolution will be useful in the testing of compounds such as drugs forthe treatment or prevention of various medical conditions or theirsafety. In some embodiments the systems and methods are used to test orevaluate potential compounds, drugs or treatments for cardiotoxicitydata. In some embodiments potential compounds, drugs or treatments aretested for cardioprotective data. In some embodiments potentialcompounds, drugs or treatments are tested for potential applicabilityfor cardiovascular treatments, including hypertension or congestiveheart failure. In some embodiments potential compounds, drugs ortreatments are tested for modulation of embryonic stem cell development,such as development into cardiomyocytes. In other embodiments thesystems and methods are used to test or assess potential drugs ortreatments for neurological data, such as beneficial or deleteriousneurological effects. As will be understood by those skilled n thepresent art, the impedance based system and methods includingmillisecond time resolution permits evaluation across multiple cellpopulations or systems that including excitation-contraction coupling.

One skilled in the art will understand embodiments of the presentinvention may be used for studying a variety of excitable cells oreffects thereon. As nonlimiting examples cells may be cardiac musclecells, neurological cells, differentiating stem cells, embryonic stemcells, adult stem cells and the like. Smooth muscle cells and skeletalmuscle cells that undergo excitation-contraction coupling may also bestudied.

In some embodiments of the present invention the excitation cycle orbeating of cardiomyocytes is studied. Cardiomyocytes are specializedmuscle cells, which make up the majority of the volume of themyocardium. The myocardium also includes epithelial cells andfibroblasts. Cardiomyocytes are arranged largely in a circumferentialand spiral orientation around the left ventricle, the chamber that pumpsblood to the systemic circulation. Atrial myocytes, also cardiomyocytes,are smaller in diameter and less structured than their ventricularcounterparts. Cardiomyocytes have five major components: cell membrane(sarcolemma) and T-tubules, for impulse conduction; sarcoplasmicreticulum, a calcium reservoir needed for contraction; contractileelements; mitochondria; and nucleus. Cellularly, cardiomyocytes may beidentified by the presence of cardiac tropnin T (TnT) and atrialnatriuretic peptide (ANP).

The functional intracellular contractile unit of cardiac muscle (likeskeletal muscle) is the sarcomere, an orderly arrangement of thickfilaments composed principally of myosin, and thin filaments containingactin. Contraction of cardiac muscle occurs by the cumulative effort ofsliding of the actin filaments between the myosin filaments toward thecenter of each sarcomere.

Cardiomyocytes may be obtained or isolated from cardiac tissue directly,may be cultured from cardiomyocyte cells or may be differentiated fromstem cells, such as embryonic stem cells. In some embodiments embryonicor adult stem cells are used. Methods for obtaining or isolatingcardiomyocytes from cardiac tissue may be found in the literature.

Since embryonic stem (ES) cells are self renewing cells in culture theycan serve as an excellent source for continuous production ofcardiomyocytes. These derived cardiomyocytes, which behave in every waylike normal cardiomyocytes isolated from the heart tissue itself,addresses the ever important supply problem and for the first timeallows for assessment of cardiac function and its modulation by leadcandidate drugs and compounds in relatively large scale. Furthermore,because the technology exists to selectively knockout or expresstrans-genes in ES cells, it provides an excellent model system to studythe role of certain genes in cardiac development and function withoutconcern of adverse affects on overall embryonic development intransgenic animals. Further, the ability to express transgenes in EScells has been utilized as a way to enrich for preparation ofcardiomyocytes that are 100% pure. For example, the gene encoding GFPhas been cloned downstream of a cardiac-specific promoter and thenintroduced into ES cells. Embryoid cells which ultimately differentiateinto cardiomyocytes express the GFP transgenes and can be easilyidentified and isolated by cell sorting techniques and therefore anenriched cardiomyocyte population can be obtained.

C. Impedance-Based Systems Having Millisecond Time Resolution

In preferred embodiments the system for monitoring impedance ofexcitable cells in vitro includes a device for monitoring cell-substrateimpedance, an impedance analyzer capable of impedance measurements atmillisecond time resolution, electronic circuitry that can engage thedevice and selectively connect two or more electrode arrays of thedevice to the impedance analyzer and a software program that controlsthe electronic circuitry and records and analyzes data obtained from theimpedance analyzer. By providing the impedance based system withmillisecond time resolution, excitation-contraction coupling cells canbe efficiently monitored and studied. Accordingly, the systems providedherein are able to identify and evaluate changes inexcitation-contraction events, which may be used for high throughputanalysis of potential therapeutics. Millisecond time resolutionmeasurements may be coupled with longer term impedance monitoring, suchas longer than seconds, hours or days. In some embodiments, long termimpedance monitoring is performed with intermittent periods of impedancemonitoring at millisecond time resolution.

Embodiments of the device for monitoring substrate impedance include anonconductive substrate having one or more individually addressableelectrode arrays fabricated thereon and one or more wells. A surface ofthe substrate may be suitable for cell attachment, wherein the cellattachment results in a detectable change in impedance betweenelectrodes within the array. Preferably, the nonconducting substrate isplanar, and is flat or approximately flat. The substrates may beconstructed from a variety of nonconductive materials known in thepresent art, including, but not limited to, silicon dioxide on silicon,silicon-on-insulator (SOI) wafer, glass (e.g., quartz glass, lead glassor borosilicate glass), sapphire, ceramics, polymer, fiber glass,plastics, e.g., polyimide (e.g. Kapton, polyimide film supplied byDuPont), polystyrene, polycarbonate, polyvinyl chloride, polyester,polypropylene and urea resin. Preferably, the substrate is biocompatiblewith excitable cells; however, materials that are not biocompatible canbe made biocompatible by coating with a suitable material, such as abiocompatible polymer or coating. Further, attachment or growth alongthe substrate or electrodes may be enhanced by precoating the substratewith a protein or compound that facilitates attachment or growth. Suchcompounds may be chosen according to techniques known in the cellularbiology arts; however, in some embodiments fibronectin is effective.Alternatively, the substrate may be chemically modified to displayreactive groups that enhance cell attachment, particularly ES cells orcardiomyocytes.

Each electrode array includes two or more electrode structures that areconstructed to have dimensions and spacing such that they can, whenconnected to a signal source, operate as a unit to generate anelectrical field in the region of spaces around the electrodestructures. Preferably the electric field is substantially uniformacross the array. An electrode structure refers to a single electrode,particularly one with a complex structure. Specifically, an electrodearray includes two electrode structures, each of which includes multipleelectrode elements, or substructures, which branch from the electrodestructure. In preferred embodiments, the electrode structures of each ofthe two or more electrode arrays of a device have substantially the samesurface area.

Each of the two electrode structures of an electrode array is connectedto a separate connection pad that is preferably located at the edge ofthe substrate. Specifically, for each of the two or more electrodearrays of the device, preferably the first of the two electrodestructures is connected to one of the two or more connection pads, andthe second of the two electrode structures is connected to another ofthe two or more connection pads. Preferably, each array of a device isindividually addressed, meaning that the electrical traces andconnection pads of the arrays are configured such that an array can beconnected to an impedance analyzer in such a way that a measuringvoltage can be applied across a single array at a given time by usingswitches (such as electronic switches).

Preferably, each electrode array of the device has an approximatelyuniform electrode resistance distribution across the entire array. By“uniform resistance distribution across the array” is meant that when ameasurement voltage is applied across the electrode structures of thearray, the electrode resistance at any given location of the array isapproximately equal to the electrode resistance at any other location onthe array. Preferably, the electrode resistance at a first location onan array of the device and the electrode resistance at a second locationon the same array does not differ by more than 30%. More preferably, theelectrode resistance at a first location on an array of the device andthe electrode resistance at a second location on the same array does notdiffer by more than 15%. Even more preferably, the electrode resistanceat a first location on an array of the device and a second location onthe same array does not differ by more than 5%. More preferably yet, theelectrode resistance at a first location on an array of the device and asecond location on the same array does not differ by more than 2%.

Preferred arrangements for electrode elements and gaps between theelectrodes and electrode buses in a given electrode array are used toallow all cells, no matter where they land and attach to the electrodesurfaces and to contribute similarly to the total impedance changemeasured for the electrode array. Thus, it is desirable to have similarelectric field strengths at any two locations within any given array ofthe device when a measurement voltage is applied to the electrode array.At any given location of the array, the field strength is related to thepotential difference between the nearest point on a first electrodestructure of the array and the nearest point on a second electrodestructure of the array. It is therefore desirable to have similarelectric potential drops across the electrode elements and across theelectrode buses of a given array. Based on this requirement, it ispreferred to have an approximately uniform electrode resistancedistribution across the whole array where the electrode resistance at alocation of interest is equal to the sum of the electrode resistancebetween the nearest point on a first electrode structure (that is thepoint on the first electrode structure nearest the location of interest)and a first connection pad connected to the first electrode structureand the electrode resistance between the nearest point on a secondelectrode structure (that is the point on the first electrode structurenearest the location of interest) and a second connection pad connectedto the second electrode structure.

Preferably, devices of the present invention are designed such that thearrays of the device have an approximately uniform distribution acrossthe whole array. This can be achieved, for example, by having electrodestructures and electrode buses of particular spacing and dimensions(lengths, widths, thicknesses and geometrical shapes) such that theresistance at any single location on the array is approximately equal tothe resistance at any single other location on the array. In mostembodiments, the electrode elements (or electrode structures) of a givenarray will have even spacing and be of similar thicknesses and widths,the electrode buses of a given array will be of similar thicknesses andwidths, and the electrode traces leading from a given array to aconnection pad will be of closely similar thicknesses and widths. Thus,in these preferred embodiments, an array is designed such that thelengths and geometrical shapes of electrode elements or structures, thelengths and geometrical shapes of electrode traces, and the lengths andgeometrical shapes of buses allow for approximately uniform electroderesistance distribution across the array.

In some preferred embodiments of impedance measurement devices,electrode structures comprise multiple electrode elements, and eachelectrode element connects directly to an electrode bus. Electrodeelements of a first electrode structure connect to a first electrodebus, and electrode elements of a second electrode structure connect to asecond electrode bus. In these embodiments, each of the two electrodebuses connects to a separate connection pad via an electrical trace.Although the resistances of the traces contribute to the resistance at alocation on the array, for any two locations on the array the traceconnections from the first bus to a first connection pad and from thesecond bus to a second connection pad are identical. Thus, in thesepreferred embodiments trace resistances do not need to be taken intoaccount in designing the geometry of the array to provide for uniformresistances across the array.

In preferred embodiments of the present invention, a device formonitoring cell-substrate impedance has two or more electrode arraysthat share a connection pad. Preferably one of the electrode structuresof at least one of the electrode arrays of the device is connected to aconnection pad that also connects to an electrode structure of at leastone other of the electrode arrays of the device. Preferably for at leasttwo arrays of the device, each of the two or more arrays has a firstelectrode structure connected to a connection pad that connects with anelectrode structure of at least one other electrode array, and each ofthe two or more arrays has a second electrode structure that connects toa connection pad that does not connect with any other electrodestructures or arrays of the device. Thus, in preferred designs of adevice there are at least two electrode arrays each of which has a firstelectrode structure that is connected to a common connection pad and asecond electrode structure that is connected to an independentconnection pad.

In some preferred embodiments of the present invention, each of theelectrode structures of an array is connected to an electrode bus thatis connected to one of the two or more connection pads of the device viaan electrically conductive trace. In preferred embodiments, each of thetwo electrode structures is connected to a single bus, such that eacharray connects to two buses, one for each electrode structure. In thisarrangement, each of the two buses connects to a separate connection padof the substrate.

The electrically conductive traces that connect a bus with a connectioncan be fabricated of any electrically conductive material. The tracescan be localized to the surface of the substrate, and can be optionallycovered with an insulating layer. Alternatively the traces can bedisposed in a second plane of the substrate. Description of arrangementsand design of electrically conductive traces on impedance measurementdevices can be found in U.S. Pat. No. 7,470,533, herein incorporated byreference for all disclosure on fabrication and design of electricallyconductive trace on substrates.

Appropriate electronic connection means such as metal clips engaged ontothe connection pads on the substrate and connectedprinted-circuit-boards can be used for leading the electronicconnections from the connection pads on the devices to externalelectronic circuitry (e.g. an impedance analyzer). Description of thedesign of cell-substrate impedance devices and their manufacture can befound in U.S. Pat. No. 7,470,533, herein incorporated by reference forall description and disclosure of the design, features, and manufactureof impedance device comprising electrode arrays.

Descriptions of electrode arrays used for impedance measurement thatapply to the devices of the present invention are described in U.S. Pat.No. 7,470,533, herein incorporated by reference for all disclosurerelating to electrode arrays (or structural units), electrodestructures, electrode materials, electrode dimensions, and methods ofmanufacturing electrodes on substrates.

Preferred electrode arrays for devices of the present invention includearrays comprising two electrode structures, such as, for example, spiralelectrode arrays and interdigitated arrays. In some preferred devices ofthe present invention, electrode arrays are fabricated on a substrate,in which the arrays comprises two electrode structures, each of whichcomprises multiple circle-on-line electrode elements, in which theelectrode elements of one structure alternate with the electrodeelements of the opposite electrode structure. Electrode arrays may beprovided in configurations, such as interdigitated, circle-on-line,diamond-on-line, concentric, sinusoidal and castellated.

Preferably, the electrode elements (or electrode structures) of an arrayof the present device of the present invention are of approximatelyequal widths. Preferably the electrode elements (or electrodestructures) of an array of the present device of the present inventionare greater than 20 microns and less than 500 microns in width, morepreferably from about 50 to about 300 microns in width.

Preferably, the electrode elements (or electrode structures) of an arrayof the present device of the present invention are approximately evenlyspaced. Preferably, the gap between electrode elements (or electrodestructures) of an array of the present device of the present inventionis less than 100 microns and more than 5 microns in width, morepreferably from about 10 to about 80 microns in width.

A device of the present invention can include one or morefluid-impermeable receptacles which serve as fluid containers or wells.Such receptacles may be reversibly or irreversibly attached to or formedwithin the substrate or portions thereof (such as, for example, wellsformed as in a microtiter plate). In another example, the device of thepresent invention includes microelectrode strips reversibly orirreversibly attached to plastic housings that have openings thatcorrespond to electrode structure units located on the microelectrodestrips. Suitable fluid container materials comprise plastic, glass, orplastic coated materials such as a ceramic, glass, metal, etc.Descriptions and disclosure of devices that comprise fluid containerscan be found in U.S. Pat. No. 7,470,533, herein incorporated byreference for all disclosure of fluid containers and fluid containerstructures that can engage a substrate comprising electrodes forimpedance measurements, including their dimensions, design, composition,and methods of manufacture.

In preferred embodiments, each electrode array on the substrate of adevice of the present invention is associated with a fluid-impermeablecontainer or receptacle, such as, for example, a well. Preferably, thedevice of the present invention is assembled to a bottomless, multiwellplastic plate or strip with a fluid tight seal. The device is assembledsuch that a single array of the substrate is at the bottom of areceptacle or well. Preferably, each array of a device is associatedwith a well of a multiwell plate. In some preferred embodiments, amultiwell device for cell-substrate impedance measurement has“non-array” wells that are attached to the substrate but not associatedwith arrays. Such wells can optionally be used for performingnon-impedance based assays, or for viewing cells microscopically.

The design and assembly of multiwell impedance measurement devices isdescribed in U.S. Pat. No. 7,470,533, and also in U.S. Pat. No.7,192,752, both herein incorporated by reference for disclosure ofmultiwell impedance measurement devices, including their design,composition, and manufacture. A device of the present inventionpreferably has between 2 and 1,536 wells and more preferably between 4and 384 wells. In some embodiments the device includes 6 wells, 16wells, 32 wells, 96 wells or 386 wells.

In some preferred embodiments, commercial tissue culture plates can beadapted to fit a device of the present invention. Bottomless plates mayalso be custom-made to preferred dimensions. Preferably, well diametersare from about 1 millimeter to about 20 millimeters, more preferablyfrom about 2 millimeters to about 8 millimeters at the bottom of thewell (the end disposed on the substrate). The wells can have a uniformdiameter or can taper toward the bottom so that the diameter of thecontainer at the end in contact with the substrate is smaller than thediameter of the opposing end.

In the system for monitoring impedance of excitable cells the impedanceanalyzer engages connection pads of one or more multi-well devices tomeasure impedance. In one embodiment of the above system, the impedanceanalyzer is capable of measuring impedance between 0.1 ohm and 10⁵ ohmin frequency range of 1 Hz to 1 MHz. The impedance analyzer ispreferably capable of measuring both resistance and reactance(capacitive reactance and inductive reactance) components of theimpedance. In a preferred embodiment of the above system, the impedanceanalyzer is capable of measuring impedance between 1 ohm and 10³ ohm infrequency range of 1.00 Hz to 300 kHz.

In preferred embodiments the impedance analyzer is capable of impedancemeasurements at millisecond time resolution. The required or desiredtime resolution may vary depending on the excitation cycle of theexcitable cell. Excitable cells having shorter excitation cycles wouldtend to require faster time resolution. In some embodiments 500millisecond time resolution is sufficient, such that at least twoconsecutive impedance measurements are between about 300 millisecondsand about 500 milliseconds apart. In preferred embodiments, impedancemeasurement at millisecond time resolution includes at least twoconsecutive impedance measurements less than 100 milliseconds apart. Insome instances the at least two consecutive impedance measurements areless than 50 milliseconds or less than 40 milliseconds apart. In someinstances the at least two consecutive impedance measurements are lessthan 20 milliseconds apart. In some instances at least two consecutiveimpedance measurements are less than 10 milliseconds apart. In someinstances millisecond time resolution includes two consecutive impedancemeasurements between 1 millisecond and 5 milliseconds, between 5milliseconds and 10 milliseconds, between 10 milliseconds and 20milliseconds, between 20 milliseconds and 40 milliseconds, or between 40milliseconds and 50 milliseconds apart. In some instances millisecondtime resolution includes at least two consecutive impedance measurementsbetween 50 milliseconds and 100 milliseconds apart. In some instancesmillisecond time resolution includes at least two consecutive impedancemeasurements between 100 milliseconds and 150 milliseconds or between150 and 300 milliseconds apart.

Millisecond time resolution of impedance-based devices provides asignificant technical step over traditional impedance measuring devices,which are typically limited to second or minute resolution. That is,substantial reconfiguration of a traditional impedance-based system isrequired to obtain millisecond time resolution. Advances in multiplesteps are required to achieve millisecond time resolution, including theimpedance measurement circuitry together with electronic switchingcircuitry. Technological advances are further required when performingmillisecond time resolution over multiple wells, such as 4 wells, 8wells, 16 wells or 96 wells. For example, if the system comprises adevice having 96 wells, the system hardware and software should becapable of, for a given time period, measuring the impedances of all 96wells with millisecond time resolution between two adjacent orconsecutive impedance measurement points for each and every well. Thus,comparing two impedance analyzers, one for impedance measurement of 96wells with millisecond resolution and another for impedance measurementof a single well with millisecond time resolution, the one for 96 wellmeasurement is effectively 96-times faster than one for a single wellmeasurement since it is required to have a capability for performingimpedance measurement for all 96 wells with millisecond time resolution.Similarly, if a 96 well device is provided, a group of wells, such as agroup of 2, 4, 6, 12, 24 or 48 wells, may be measured in millisecondtime resolution. Preferably, the time difference between two adjacentmeasurement points for each and every given well is less than 500milliseconds. More preferably, the time difference between two adjacentmeasurement points for each and every given well is less than 300milliseconds. Still more preferably, the time difference between twoadjacent measurement points for each and every given well is less than100 milliseconds. Still more preferably, the time difference between twoadjacent measurement points for each and every given well is less than40 milliseconds. Still more preferably, the time difference between twoadjacent measurement points for each and every given well is less than20 milliseconds. Still more preferably, the time difference between twoadjacent measurement points for each and every given well is less than10 milliseconds.

A number of improvements in the impedance measurement circuitry,electronic switching circuitry, communication between impedancemeasurement circuitry and software can be used to achieve suchmillisecond time resolution. For instance, improvement likely requiresthe use of fast processing electronic chips for analogue-to-digitalconversion, for parallel digital signal processing and data calculationwith field-programmable gate array (FPGA) and for fast communicationbetween the impedance measurement circuitry and software. Anotherexample of improvements includes the use of multiple analogue-to-digital(AD) conversion channels so that analog electronic signals from multiplechannels can be converted to digital signals simultaneously. Suchparallel AD conversion is important, particular for the system havingmultiple wells, each of which's measurement time resolution is requiredto be in the millisecond resolution. And, a very important is to replaceprevious working mode of “measurement of one-well's impedance at a time”with a mode of “measurement of multiple-wells' impedances at a time”. In“one-well at a time” mode, when the software issue a command formeasuring one well's impedance, the measurement circuitry would performthe measurement for one well including signal generation to the well,converting the voltage signal and the electric current signal for thewell to digital signal, digitally processing the signals to do impedancecalculation and sending the well's impedance data to the computer overthe communication line between the impedance measurement circuitry andthe computer. The system will not perform any measurement for anotherwell until the completion of the measurement of this well and untilreceiving another command for the measurement of another well. In“multiple-wells at a time” mode, the software would issue a command formeasuring multiple wells' impedances. The measurement circuitry wouldsimultaneously or nearly simultaneously perform signal conversion,signal processing and impedance calculation for multiple wells. Themultiple impedance data for the multiple wells would be sent over thecommunication lines to the computer sequentially with one well's data atthe same time or simultaneously with more than one well's data beingsent at a time. In this “measurement of multiple-wells' impedances at atime” mode, the system may be performing multiple tasks simultaneously,for example, while one well's impedance data is being measured andcalculated, another well's impedance data may be communicated and sentover the communication lines to the computer.

With millisecond time resolution for impedance measurement, it becomespossible to resolve individual beating cycles of cardiomyocytes culturedon electrodes. Whilst theoretically one needs at least two data pointsfor each beating cycle, in practice more than 2 data points are neededfor each beating cycle. For example, if cells have a beating rate of 60beats per minute, i.,e, one beat per second. It would be preferred tohave a time resolution of at least 200 milliseconds so that each beatingcycle consists of 5 data points. More preferably, the measurement timeresolution is 100 milliseconds. Still more preferably, the timeresolution is 50 milliseconds or less.

One skilled in the art will understand that the cell-substrate impedancemeasurement or monitoring system with millisecond time resolution can beused to efficiently and simultaneously perform multiple assays by usingcircuitry of the device station to digitally switch from recording frommeasuring impedance over an array in one well to measuring impedanceover an array in another well. Similarly, groups of wells may bemonitored simultaneously and switching between occur between designatedgroups. In one embodiment of the above system, the system under softwarecontrol is capable of completing an impedance measurement for anindividual well at a single frequency within milliseconds, such as lessthan 100 milliseconds, less than 40 milliseconds, less than 20milliseconds, less than 10 milliseconds or between 1 millisecond and 40milliseconds. In some embodiments the user may choose the frequency ofmeasurement for millisecond time resolution.

A multiple-well cell-substrate impedance measuring device in a system ofthe present invention can be any multiple-well cell-substrate impedancemeasuring device in which at least two of the multiple wells comprise anelectrode array at the bottom of the well, and in which at least two ofthe multiple wells comprise an electrode array are individuallyaddressed. In one embodiment of the above system, the multi-well devicetakes the form of a specialized microtiter plate which hasmicroelectronic sensor arrays integrated into the bottom of the wells.

A device used in a system of the present invention, when connected to animpedance analyzer, can measure differences in impedance values thatrelate to cell behavior. For example, a cell-substrate impedancemeasuring device used in a system of the present invention can measuredifferences in impedance values when cells are attached to the electrodearray and when cells are not attached to the electrode array, or candetect differences in impedance values when the number, type, activity,adhesiveness, or morphology of cells attached to theelectrode-comprising surface of the apparatus changes. Further, by usingmillisecond time resolution differences in impedance may be detected ormonitored that relate to excitation-contraction coupling, including thebeating of cardiomyocytes or stem cells differentiating intocardiomyocytes, and the signaling between neurological cells. Impedancemonitoring of the excitation cycle of excitable cells may be determinedand monitored before, during or after adding a test compound, which issuspected of affecting the excitation cycle. Thus, by monitoring theexcitation cycle of the excitable cell before, during or after adding atest compound the system provides data corresponding to the potentialaffect of the compound on the cardiovascular system, the heart, thenervous system, and the like. In some embodiments monitoring theexcitation cycle of the cell before, during or after adding a compoundprovides cardiotoxicity data useful in drug screening.

In some embodiments a device station or electromechanical apparatus orassembly capable of interfacing multiwell devices can include one ormore platforms or one or more slots for positioning one or moremultiwell devices. The one or more platforms or one or more slots cancomprise sockets, pins or other devices for electrically connecting thedevice to the device station. The device station or electromechanicalapparatus or assembly capable of interfacing multiwell devicespreferably can be positioned in a tissue culture incubator during cellimpedance measurement assays. It can be electrically connected to animpedance analyzer and computer that are preferably located outside thetissue culture incubator.

The device station or electromechanical apparatus or assembly capable ofinterfacing multiwell devices includes electronic circuitry that canconnect to the impedance monitoring device and an impedance analyzer andelectronic switches that can switch on and off connections to each ofthe two or more electrode arrays of the multiwell devices used in thesystem. The switches of the device station or electromechanicalapparatus or assembly capable of interfacing multiwell devices arecontrolled by a software program, each of which has been improved toprovide millisecond time resolution. The software program directs thedevice station to connect arrays of the device to an impedance analyzerand monitor impedance from one or more of the electrode arrays. Duringimpedance monitoring, the impedance analyzer can monitor impedance atone frequency or at more than one frequency. Preferably, impedancemonitoring is performed at more than one time point for a given assay,and preferably, impedance is monitored using at least two time points.The device station can connect individual arrays of a device to animpedance analyzer to monitor one, some, or all of the arrays of adevice for a measurement time point. In some preferred embodiments ofthe present invention, the device station software is programmable todirect impedance monitoring of any of the wells of the device thatcomprise arrays at chosen time intervals.

The software of the impedance monitoring system can also store anddisplay data. Data can be displayed on a screen, as printed data, orboth. Preferably the software can allow entry and display ofexperimental parameters, such as descriptive information including cellstypes, compound concentrations, time intervals monitored, etc.

Preferably, the software can also analyze impedance data. In preferredembodiments, the software can calculate a cell index (CI) for one ormore time points for one or more wells of the multiwell device. In somepreferred embodiments, the software can calculate a cell change index(CCI) from impedance measurements of one or more wells of the multiwelldevice. The software can preferably generate plots of impedance data andimpedance values, such as but not limited to CI or CCI, with respect totime. The software may perform other analysis as well, such as calculatecell number from CI, generate dose-response curves based on impedancedata, calculate IC values based on impedance values, and calculatekinetic parameters of the excitation cycle cell based on impedancevalues and impedance value curves. In some embodiments the beating cycleof a cardiomyocyte population is determined, which may includeinitiation and decay of individual beats. Peaks may be derived from thedetection of vectors associated with initiation of beating or beatingdecay. Peaks may be derived with other methods. In further embodimentsthe change in beating cycle of a cardiomyocyte population is determinedin response to a stimulus such as a pharmacological agent. The softwareof the impedance monitoring system can also store and display analysesof the data, such as calculated impedance values and kinetic parametersderived therefrom, Data can be displayed on a screen, as printed data,or both. Data may be stored on a hard drive for exportation intocompatible programs for further analysis or data storage

Further information regarding how to calculate a cell index, cell changeindex, normalized cell index, and delta cell index may be found in U.S.patent application Ser. No. 11/235,938, and U.S. Pat. No. 7,470,533, thecontents of which are herein incorporated by reference with respect tothe cell index, cell index number, cell change index, and cell changeindex number. However each is briefly summarized.

The cell index obtained for a given well reflects: 1) how many cells areattached to the electrode surfaces in this well, 2) how well cells areattached to the electrode surfaces in the well. In this case, a zero ornear-zero “cell index or cell number index” indicates that no cells orvery small number of cells are present on or attached to the electrodesurfaces. In other words, if no cells are present on the electrodes, orif the cells are not well-attached onto the electrodes cell index=0. Ahigher value of “cell number index” indicates that, for same type of thecells and cells under similar physiological conditions, more cells areattached to the electrode surfaces. Thus Cell Index is a quantitativemeasure of cell number present in a well. A higher value of “cell index”may also indicate that, for same type of the cells and same number ofthe cells, cells are attached better (for example, cells spread outmore, or cell adhesion to the electrode surfaces is stronger) on theelectrode surfaces.

A “Normalized Cell Index” at a given time point is calculated bydividing the Cell Index at the time point by the Cell Index at areference time point. Thus, the Normalized Cell Index is 1 at thereference time point. Normalized cell index is cell index normalizedagainst cell index at a particular time point. In most cases in thepresent applications, normalized cell index is derived as normalizedrelative to the time point immediately before a compound addition ortreatment. Thus, normalized cell index at such time point (immediatelybefore compound addition) is always unit one for all wells. One possiblebenefit for using such normalized cell index is to remove the effectfrom difference in cell number in different wells. A well having morecells may produce a larger impedance response following compoundtreatment. Using normalized cell index, it helps to remove suchvariations caused by different cell numbers.

A “delta cell index” at a given time point is calculated by subtractingthe cell index at a standard time point from the cell index at the giventime point. Thus, the delta cell index is the absolute change in thecell index from an initial time (the standard time point) to themeasurement time.

The time-dependent cellular response (including cardiotoxicity response)may be analyzed by deriving parameters that directly reflect the changesin cell status. For example, time dependent cellular response may beanalyzed by calculating the slope of change in the measured impedanceresponses (that is equivalent to the first order derivative of theimpedance response with respect to time, impedance response here can bemeasured impedance data or derived values such as cell index, normalizedcell index or delta cell index). In another example, the time-dependentcellular responses (including cardiotoxic responses) may be analyzed fortheir higher order derivatives with respect to time. Such high orderderivatives may provide additional information as for how cellsresponding to different compounds and as for the mechanisms of compoundaction.

Thus CCI is the normalized rate of change in cell index. CCI values canbe used to quantify the cell status change. For cells in an exponentialgrowth under regular cell culture condition, the cell index determinedby a cell-substrate impedance monitoring system described herein isexpected to be a proportionate measure of the cell number in the wellsince the cell morphology and average extent of cell adhesion to theelectrode surfaces among the whole cell population do not exhibitsignificant changes over time.

After measuring and recording time dependent impedance data forexcitable cells cultured on electrodes, various methods can be used fordetermining beating cycle peaks. For example, time dependent impedancevalues or cell index values for a well are analyzed by deriving theirfirst order derivatives and second order derivatives using numericalmethods. The beating cycle peaks are those data points where the firstorder derivatives of impedance values or cell index values are zero orclose to zero in its absolute value. If the beating cycle peak is apositive peak (i.e. peak corresponds to a maximum value in measuredimpedance or cell index over the beating cycle), then the peak wouldcorrespond to the data points where the second order derivatives of theimpedance values or cell index values is negative and where the firstorder derivatives of the impedance values or cell index values is zeroor close to zero in its absolute value. If the beating cycle peak is anegative peak (i.e. peak corresponds to a minimum value in measuredimpedance or cell index over the beating cycle), then the peak wouldcorrespond to the data points where the second order derivatives of theimpedance values or cell index values is positive and where the firstorder derivatives of the impedance values or cell index values is zeroor close to zero in its absolute value. In another example, the methodof determining a beating cycle peak is to search for the data pointwhere the trend of the data changes direction from “increasing” to“decreasing” with time (for a positive peak), or from “decreasing” to“increasing” (for a negative peak). After the identification of thebeating cycle peaks, the impedance or cell index values at such peaktime points are the magnitude or amplitude of the beating cycle peaks.

After determining the beating cycle peaks, various methods can be usedto calculate the beating rate. The beating rate can be determined bydividing a unit time (e.g., 1 minute) by the time period between twoadjacent peaks. For example, if two adjacent peaks are separated by 500milliseconds, then the beating rate for this 500 milliseconds can bedetermined as 120 beats per minute. Another method is simply to counthow many peaks in a unit time by software. For example, if there are 2peaks in one second interval, then the beating rate should be 2 beatsper second, or 120 beats per minute.

D. Assessing or Quantifying Excitable Cells, ExcitationContraction-Coupling and Cardiomyocytes In Vitro

In another aspect of the present invention a method for assessing orquantifying excitable cells in vitro is provided. The methods includingdetecting or monitoring excitable cells using impedance monitoring withmillisecond time resolution. In preferred embodiments, the methodincludes providing the impedance based system having millisecond timeresolution system, adding excitable cells to one or more wells of thedevice, monitoring impedance of the one or more wells with millisecondtime resolution, and resolving individual cycles of the excitable cells.In some embodiments monitoring impedance further includes determiningcell indices from impedance values, calculating average rate ofexcitation events per unit time (such as beats per unit time when usingcardiomyocytes), determining amplitude intensity or average amplitudeintensity in a unit time, and/or determining the length of time betweenexcitation cycles (or beating cycles). These additional determinationsmay be compared to further assess excitable cells.

The methods have particular utility with cells that are known orsuspected to be capable of or undergo excitation-contraction coupling,such as cardiomyocytes. Thus the methods may monitor, detect ordetermine excitation contraction-coupling of excitable cells. Further,even minor variations in excitation-contraction coupling may bedetected.

Preferably impedance measurements are performed with millisecond timeresolution. In some embodiments, two consecutive impedance measurementsare performed at less than 300 milliseconds apart. In furtherembodiments, at least two consecutive impedance measurements areperformed less than 100 seconds apart. In further embodiments, at leasttwo consecutive impedance measurements are performed less than 40milliseconds apart. In further embodiments, at least two consecutiveimpedance measurements are performed less than 10 milliseconds apart. Insom embodiments at least two impedance measurements are between 1millisecond and 50 milliseconds apart.

Cardiomyocytes are a subset of cells that are involved inexcitation-contraction coupling. Embodiments of the present inventioninclude applications for studying cardiomyocyte activity, function andits modulation. By monitoring the continuous rhythmic beating ofcardiomyocytes, methods of the present invention allow the study ofcardiomyocytes themselves as well as screening for pharmacologicalagents which may alter cardiomyocyte rhythmic beating. Thus the methodspermit screening of compounds or conditions that affect cardiomyocytefunction or induce cardiotoxicity. Specifically, methods may assess orquantify cardiomyocyte beating in vitro using impedance-based technologyhaving millisecond time resolution. In preferred embodiments, themethods are based on quantification in real time of the rhythmic changesin cardiomyocyte morphology as a result of the excitation contractioncoupling of the electrically excitable cardiomyocytes growing onmicroelectrodes' surface in E-PLATES (ACEA Biosciences, Inc. San Diego,Calif.). Thus quantification of the rhythmic changes in cardiomyocytemorphology is achieved via the measurement of electrode impedance, whichmay be performed with millisecond time resolution. In summary, somemethods provide a cellular cardio-gram, which can provide incisiveinformation about the status of cardiomyocytes especially upon treatmentwith pharmacological agents.

Cardiomyocytes used in the present invention may be any known orexperimental cardiomyocyte population. Human cardiomyocytes will beparticularly desirable when studying the effect of pharmological agentsfor human use; however, cardiomyocytes may also be murine or mouse, rat,rabbit, dog, and the like. Further, the cardiomyocytes may be primarycardiomyocytes isolated from cardiac tissue or may be culturedcardiomyocytes, which are cultured over time. In some embodiments, EScells destined to differentiate into cardiomyocytes are used. Isolatedprimary cardiomyocytes as well as ES-derived cardiomyocytes retain theability to beat in culture. Thus, these cells provide a desirable modelsystem to study cardiomyocyte function in vitro.

Cardiomyocytes have an innate ability to undergo mechanotransduction. Inother words, the spontaneous force generated from the beatingcardiomyocyte is translated to intracellular biochemical signals.Membrane receptors such as integrins, ion channels and other proteinshave been shown to play a crucial role in cardiac mechanotransductionand lead to a continuous and rhythmic dynamics of the cardiac actincytoskeleton and morphology. Because the impedance-based system cansensitively and precisely detect transient changes in morphology andadhesive capacity of the cells, it can be used to monitor cardiacfunction in vitro. Further, the development of millisecond timeresolution permits impedance monitoring of the cardiomyocyte beatingcycle as well as fluctuations, even minor alterations, in response toexternal stimuli.

Exemplary steps involved in using an impedance-measurement system formeasurement of cardiomyocte function include: providing a system formonitoring impedance of excitable cells, in vitro, which is capable ofmillisecond time resolution, optionally coating wells of the device witheither fibronectin or other matrix proteins; seeding either embryonicstem cells (ES cells) of mammalian origin or primary cardiomyocytes atsufficient seeding densities into the wells of the device; allowing thecells to attach and spread; after a period of time unique to ES-derivedcardiomyocytes or primary cardiomyocytes, monitoring cardiomyocytebeating using the impedance-monitoring system to monitor electrodeimpedance by using millisecond kinetic readout to resolve the individualbeat cycles of the cells.

The millisecond kinetic readout requires the impedance measurementsystem be capable of obtaining impedance measurement data at millisecondtime resolution. Preferably, the time difference between two consecutiveimpedance measurements for a well is within the range of milliseconds(e.g., less than 300 millisecond, less than 100 milliseconds, less than50 milliseconds, less than 20 milliseconds, less than 10 milliseconds,or less than or about 1 millisecond). Due to the fast nature of beatingcycles of cells, millisecond kinetic readout is required for properresolution. Thus, the time resolution for the impedance measurementshould allow the system to perform measurement using at least two timepoints for each beat cycle, or at more than two points for each beatingcycle. In some embodiments, three, four, five or even ten, or fifteen ortwenty or more time points within each beating cycle are obtained.

Embodiments of the present invention include methods of assessing orquantifying cardiomyocytes include providing the system for monitoringimpedance of excitable cells in vitro, which includes millisecond timeresolution, adding cardiomyocytes to one or more wells, monitoringimpedance of the one or more wells, such as using millisecond timeresolution, and resolving excitation cycle or beating cycle of thecardiomyocytes In further embodiments, the method also includesdetermining cell indices from impedance values, optionally calculatingaverage rate of beats per unit time, average amplitude intensity in aunit time as well as the average length of time between the beats. Inpreferred embodiments, millisecond time resolution permits measurementof at least two consecutive impedance measurements within a beatingcycle. In some embodiments at least three consecutive impedancemeasurements are performed with millisecond time resolution.

Isolated primary cardiomyocytes as well as ES-derived cardiomyocytesretain the ability to beat in culture. These cells provide an excellentmodel system to study cardiomyocyte function in vitro, especially withregards to cardiotoxicity. A number of cardiotoxic drugs are known toaffect certain heart channels, such as the ERG channels, that areinvolved in excitation-contraction coupling of cardiomyocytes.Cardiomyocytes have an innate ability to undergo mechanotransduction,that is that the spontaneous force generation of the beatingcardiomyocyte is translated to intracellular biochemical signals.Membrane receptors such as integrins, ion channels and other proteinshave been shown to play a crucial role in cardiac mechanotransductionand lead to a continuous and rhythmic dynamics of the cardiac actincytoskeleton and morphology. Because the impedance-based system cansensitively and precisely detect transient changes in morphology andadhesive capacity of the cells, thus it can be used to monitorcardiomyocyte beating in vitro.

Another exemplary method for impedance based measurement ofcardiomyocyte beating includes: providing a single-well or multi-welldevice that comprise microelectrode arrays in well(s) of the device,which can be used for monitoring cell-substrate impedance; optionallycoating wells of the device with either fibronectin or other matrixproteins; seeding either embryonic stem cells (ES cells) of mammalianorigin or primary cardiomyocytes at specific seeding densities to thewells of the device; allowing the cells to attach and spread; after aspecified period of time unique to ES-derived cardiomyocytes or primarycardiomyocytes, monitoring cardiomyocyte beating using theimpedance-monitoring system to monitor electrode impedance by usingmillisecond kinetic readout to resolve the individual beat cycles of thecells.

In another exemplary embodiment for measurement of cardiomyocte functionthe method includes: optionally coating E-PLATES with either fibronectinor other matrix proteins; seeding either ES cells of mammalian origin orprimary cardiomyocytes at specific seeding densities; allowing the cellsto attach and spread; after a specified period of time unique toES-derived cardiomyocytes or primary cardiomyocytes, monitoringcardiomyocyte beating using an impedance-based system by usingmillisecond kinetic readout to resolve the individual beat cycles of thecells.

Dynamic monitoring of cardiomyocyte beating is based on quantificationin real time of the rhythmic changes in cardiomyocyte morphology as aresult of the excitation contraction coupling of the electricallyexcitable cardiomyocytes growing on microelectrodes' surface ofE-PLATES. The quantification of the rhythmic changes in cardiomyocytemorphology is achieved via the fast, millisecond resolution andcontinuous measurement of electrode impedance. The method essentiallyprovides a cellular cardio-gram which can provide incisive informationabout the status of cardiomyocytes especially upon treatment withpharmacological agents. An exemplary method includes providing a devicefor measuring cell-substrate impedance operably connected to animpedance analyzer, wherein the device includes at least one welloptionally coated with fibronectin to expedite attachment; adding cellsto the at least one well, where the cells can be mouse or human or othermammalian ES cells or adult stem cells destined to differentiate intocardiomyocytes or primary cardiomyocytes isolated directly from theheart of an experimental system including mice, rats, rabbits or dog;monitoring impedance of the at least one well at time intervals over aperiod of time and optionally determining cell indices from impedancevalues; optionally calculating average rate of beats per unit time,average amplitude intensity in a unit time as well as the average lengthof time between the beats.

Certain cardiotoxic drugs can illicit their effect by affecting themorphological aspects of cardiomyocyte morphology. For example, it iswell known that compounds such as β-2 adrenergic receptor agonists caninduce morphological changes resulting in an elongated cardiomyocytemorphology, otherwise known as hypertrophy. Morphological changes canoccur immediately in the order of minutes as with certain GPCR agonistsor can be of longer duration detectable over several days. The timeresolution of the impedance-based system can be used to distinguishbetween different kinds of morphological effects. The steps involved inusing an impedance-monitoring system for measurement of morphologicalmodulation of cardiomyocytes may include: optionally coating E-PLATESwith either fibronectin or other matrix proteins; seeding eitherembryonic stem cells (ES cells) of mammalian origin, mammalian adultstem cell-derived cardiomyocytes or primary cardiomyocytes isolateddirectly from mammalian heart tissue at specific seeding densities tothe wells of the device; allowing the cells to attach and spread;monitoring cardiomyocyte viability over time using theimpedance-monitoring system at prespecified intervals of time for 12,24, 48, 72 hours or longer; at certain times after cell seeding, treatthe cell with agents that may cause morphology changes at one or moreconcentration; using the vehicle that the agent is dissolved in as acontrol; continuing to monitoring the cardiomyocytes at 1 minuteintervals for at least 1-2 hours to capture any immediate morphologicalchanges and continue to monitor at 30 minutes intervals of time foradditional 12, 24, 48, 72 hours or longer to detect long termmorphological changes; and quantifying the extent of morphologicalchange by normalizing the cell index values immediately prior to agentaddition and determine the normalized cell index at a given time pointafter agent addition. The extent of morphological change can beexpressed as EC-50 value which quantifies the activity of the agent withrespect to the cardiomyocyte shape changes.

Once regular cardiomyocyte beating can be effectively monitoredcompounds may be added to the wells to assess whether or not beatingfrequency is altered, beating amplitude is altered and the like. Thus,compounds that adversely affect the normal cardiomyocyte beating aresuspected of being cardiotoxic. However, cardioprotective agents canalso be assessed by adding an agent suspected of being cardioprotectiveto the well and inducing a cardiotoxic response.

Accordingly, methods of identifying or screening for potential agentsthat modulate ES-derived cardiomyocyte beating or primary cardiomyocytebeating by monitoring and measuring the excitation-contraction couplingof cardiomyocytes upon treatment with pharmacological agents are alsoprovided. The methods may include providing the system for monitoringimpedance at millisecond resolution of excitable cells in vitro, addingES cells or primary cardiomyocytes to two or more wells, adding a testagent suspected of modulating ES-derived or primary cardiomyocytebeating to a first well to provide a test well and either no agent or acontrol agent to a second well to form a control well, monitoringimpedance of the two or more wells at different or similar timeintervals using millisecond time resolution at least in part, andoptionally determining cell indices from impedance values, generating animpedance-based curve or optionally a cell index curve for each of thetest well and control well, and comparing the impedance-based curves oroptionally the cell index curves between the test well and the controlwell. If significantly different, the test agent is concluded tomodulate cardiomyocyte function. In preferred embodiments, theimpedance-based curves may be direct measurement of cardiomyocyteexcitation-contraction coupling; however, the methods may also includelong term monitoring of the viability and or morphology of thecardiomyocytes. The methods may show whether the test agent increasescardiomyocyte beating, which is indicative of increased heart rate, orwhether the cardiomyocyte beating is slowed, which is indicative oflowered heart rate.

In another exemplary embodiment a method for screening for modulators ofcardiomyocyte beating using an impedance-measurement system in vitro mayinclude: providing a single-well or multi-well device that comprisemicroelectrode arrays in well(s) of the device, which can be used formonitoring cell-substrate impedance; optionally coating wells of thedevice with either fibronectin or other matrix proteins; seeding eitherembryonic stem cells (ES cells) of mammalian origin, mammalian adultstem cell-derived cardiomyocytes or primary cardiomyocytes at specificseeding densities to the wells of the device; allowing the cells toattach and spread; after a specified period of time unique to ES-derivedcardiomyocytes or primary cardiomyocytes, monitoring cardiomyocytebeating using the impedance-monitoring system to monitor electrodeimpedance by using millisecond kinetic readout to resolve the individualbeat cycles of the cells; and adding the pharmacological agents at oneor more doses and continue monitoring the cardiomyocyte beatingfrequency. Preferably, the step of monitoring impedance of beating isperformed immediately prior to addition of pharmacological agent, inorder to obtain a baseline of the cardiomyocyte beating frequency usingfast measurement software and hardware to ensure milli-second kineticreadout signals.

In another exemplary embodiment, a method of screening for modulators ofcardiomyocyte beating using an impedance-based system in vitro includesseeding cardiomyocytes, in E-PLATES exactly as described in the previousexemplary embodiment; however, prior to addition of pharmacologicalagent, obtaining a baseline of the cardiomyocyte beating frequency usingthe impedance measurement system with millisecond time resolution; andadding the pharmacological agents at one or more doses and continuemonitoring the cardiomyocyte beating frequency.

It is well established that certain pharmacological treatments anddisease conditions can result in cardiac hypertrophy or atrophyculminating in changes in the morphology of cardiomyocyte. Cellsubstrate impedance can be used to precisely measure and quantify thesechanges in cell morphology and shape. Certain treatments can also affectthe differentiative process of ES cells to cardiomyocytes which mayinvolve specific morphological and adhesive changes. In some embodimentsthe present invention is directed to a method to screen for potentialagents that may modulate the morphology of ES-derived cardiomyocyte,adult stem cell-derived cardiomyocyte or primary cardiomyocyte or itsdifferentiation. The agent may include but is not limited to compounds,drugs, peptides, proteins, antibodies, siRNA, shRNA, miRNA, cDNA, lipidsand any combination thereof. The method includes providing a device formeasuring cell-substrate impedance operably connected to an impedanceanalyzer, wherein the device includes at least two wells; adding EScells, adult stem-cell derived cardiomyocytes or primary cardiomyocytesto at least two wells; monitoring impedance of the at least two wells atdifferent or similar time intervals over a period of time and optionallydetermining cell indices from impedance values; generating animpedance-based curve or optionally a cell index curve for each of theat least one known factor and the control; comparing the impedance-basedcurves or optionally the cell index curves between the at least oneknown biologically active agent well and the control well; theimpedance-based curves could be direct measurement of changes in cellmorphology and if significantly different, concluding that thebiologically active agent modulates cardiomyocyte function. Optionally,impedance-based curves or optionally cell index curves are used tocalculate the compound dose-dependent changes in cardiomyocytemorphology and generate an EC-50 value for the potency of the compound.

The method of the present invention is to devise a label-free cell-basedassay system for continuous monitoring of cardiomyocyte viability, therhythmic beating of cardiomyocytes and cardiomyocyte morphology anddifferentiation and to screen for pharmacological agents which maymodulate these processes and induce cardiotoxicity. In some embodiments,viability is monitored based on long term impedance monitoring ofcardiomyocytes seeded in microelectronic plates (E-Plates). Viable cellswill continue to generate impedance signal and any changes in viability,especially due to cytotoxic or cardiotoxic drugs will be reflected bychanges in impedance. In one respect, the method includes providing adevice for measuring cell-substrate impedance operably connected to animpedance analyzer, wherein the device includes at least one welloptionally coated with fibronectin to expedite attachment; adding cellsto the at least one well, where the cells can be mouse or human or othermammalian ES cells or adult stem cells destined to differentiate intocardiomyocytes or primary cardiomyocytes isolated directly from theheart of an experimental system including mice, rats, rabbits or dog;monitoring impedance of the at least one well over a period of time andoptionally determining cell index from impedance values; In anotheraspect, the method includes providing a device for measuringcell-substrate impedance operably connected to an impedance analyzer,wherein the device includes at least two wells optionally coated withfibronectin to expedite attachment; adding cells to the at least twowells, where the cells can be mouse or human or other mammalian ES cellsor adult stem cells destined to differentiate into cardiomyocytes orprimary cardiomyocytes isolated directly from the heart of anexperimental system including mice, rats, rabbits or dog; monitoringimpedance of the at least two wells over a period of time and optionallydetermining cell index from impedance values. Treating at least one wellwith an agent; where the agent could include but is not limited to acompound, peptide, protein, antibody, siRNA, shRNA, lipid or anycombination of thereof and the other well is treated with an appropriatecontrol; continue monitoring of control and treated well over a periodof time preoptimized for the experiment of interest; concluding that thefactor may affect cell viability if the impedance or alternatively thecell index of the treated well is significantly different than theimpedance or cell index of the treated well.

In other preferred embodiments short term measurement using millisecondtime resolution is combined with longer term impedance monitoring, suchas over seconds, hours, days and the like. Certain cardiotoxic drugs candirectly affect the viability of cardiomyocytes. Thus, long termimpedance monitoring may be used to assess loss of viability ofcardiomyocytes. An exemplary long term method to assess loss ofviability includes: optionally coating E-PLATES with either fibronectinor other matrix proteins; seeding either embryonic stem cells (ES cells)of mammalian origin, mammalian adult stem cell-derived cardiomyocytes orprimary cardiomyocytes isolated directly from mammalian heart tissue atseeding densities to the wells of the device; allowing the cells toattach and spread; monitoring cardiomyocyte viability over time usingthe impedance-monitoring system to monitor electrode impedance atpre-specified intervals of time for specified length of time such as 12,24, 37, 48, 72 hours or longer; at certain time after cell seeding,treating the cell with a cytotoxic agent at one or more concentration;using the vehicle that the agent is dissolved in as a control; continuemonitoring the cardiomyocytes at pre-specified intervals of time forspecified length of time such as additional 12, 24, 48, 72 hours orlonger, quantify the extent of cardiotoxicity by normalizing the cellindex values immediately prior to agent addition and determining thenormalized cell index at a given time point after agent addition;alternatively the rate of cytotoxicity can also be quantified for agiven time period after compound addition for a given agentconcentration or a group of concentrations. The extent of cytotoxicitycan be expressed as IC-50 value which quantifies the activity of theagent with respect to the cardiomyocytes.

In another aspect of the present invention, direct optical monitoring ofcardiomyocytes is used to quantify and measure the beating ofcardiomyocytes. The method includes providing a device for opticallymonitoring cells and monitoring cell morphology operably connected to anoptical measurement system, where the device includes at least two wellsoptionally coated with fibronectin to expedite attachment; adding cellsto the at least two wells, where the cells can be mouse or human orother mammalian ES cells destined to differentiate into cardiomyocytesor primary cardiomyocytes isolated directly from the heart of anexperimental system including mice, rats, rabbits or dog; opticallymonitoring the cells of at least two wells at time intervals over aperiod of time via the optical measurement system; optionallycalculating average rate of beats per unit time, average amplitudeintensity in a unit time as well as the average length of time betweenthe beats. The device for such optical measurement of cells may includemicrotiter plates. The optical system may include optical magnificationinstrument such as microscope, optical CCD camera, optical-signalprocessing algorithm to quantify cell beating and to derive cell-beatingparameters (such as calculating average rate of beats per unit time,average amplitude intensity in a unit time as well as the average lengthof time between the beats) based on cell morphology images.

On skilled in the art will recognize that any agent may be tested. Theagent may be a biologically active agent. Exemplary test agents includecompounds, drugs, peptides, proteins, antibodies, antibody fragments,siRNA, shRNA, miRNA, cDNA, lipids and any combination thereof.Optionally, impedance-based curves or optionally cell index curves areused to calculate average rate of beats of cardiomyocytes per unit time,average amplitude intensity in a unit time as well as the average lengthof time between beats, comparison of these optionally derived parametersis made between the test well and the control well, and if significantdifferences exist, one may optionally conclude that the test agentmodulates cardiomyocyte functions.

The study of cardiomyocyte function and modulation has particularutility in the biomedical arts. For example, the methods of the presentinvention may be used to assess whether a potential agent iscardiotoxic, cardioprotective and the like. If normal cardiomyocytebeating is altered, the result likely demonstrates the potential agentis cardiotoxic. Such information would permit effective screening priorto testing in live animals or in humans. Further, by couplingmillisecond time resolution over extended time periods, the methods mayassess short term and long term effects on cardiomyocytes. Since it iswell established that certain pharmacological treatments and diseaseconditions can result in cardiac hypertrophy or atrophy culminating inchanges in the morphology of cardiomyocyte, the systems and methods areable to precisely measure and quantify these changes in cell morphologyand shape. Thus the methods and systems have particular utility in theareas of biomedical research.

In some embodiments, potential agents are tested to assess whether theyare cardioprotective, or protect against adverse affects. In suchembodiments the agent suspected of being cardioprotective is added tothe cardiomyocytes prior to, simultaneously or after adding acardiotoxic agent or a cardiotoxic event. Thus, a cardioprotective agentmay prevent abnormal shifts in beating cycle, which would naturallyoccur in the presence of a cardiotoxic agent. In further embodiments,cardiomyocytes having abnormal beating cycles are treated with potentialagents to assess whether normal beating may be induced, such as throughadministration of a cardioprotective pharmacological agent.

Certain treatments can also affect the differentiative process of EScells to cardiomyocytes which may involve specific morphological andadhesive changes. In another aspect of the present invention a method isprovide to screen or potential agents that may modulate the morphologyof ES-derived cardiomyocyte, adult stem cell-derived cardiomyocyte orprimary cardiomyocyte or its differentiation.

Impedance-based curves and optionally cell index curves may be used tocalculate average rate of beats per unit time, average amplitudeintensity in a unit time as well as the average length of time betweenthe beats. Significant difference between measurements or data of a testwell and a control well support a finding that the biologically activeagent within the test well modulates cardiomyocyte function.

E. Assessing the Developmental or Functional Consequence of GeneKnockout and Transgene Expression in Embryonic Stem Cell-DerivedCardiomyocytes

Embodiments of the present invention also include assessing thedevelopmental consequence of genetically manipulated ES cells or theirderived cardiomyocytes. ES cells offer a suitable experimental modelsystem that is amenable to genetic manipulation. Therefore, specificgenes can be targeted in knockout experiment as well as genes can beexpressed in a developmental or stage specific manner under the controlof special promoters. The impedance-based measurement system withmillisecond time resolution can be used to evaluate the role of thesegenes in cardiac development and or function.

Steps involved in assessing the developmental and functional effect ofgene knockout or transgene expression may include: obtaining ES cellsharboring specific knockout of genes or which expresses a particulartransgene; provide the system for monitoring impedance of excitablecells having millisecond time resolution; optionally coating wells ofthe device with either fibronectin or other matrix proteins; seeding theembryonic stem cells (ES cells) of mammalian origin at desired seedingdensities into the wells of the device; allowing the cells to attach andspread; after a time period unique to ES-derived cardiomyocytes orprimary cardiomyocytes, and monitoring cardiomyocyte beating using theimpedance-monitoring system using millisecond kinetic readout to resolvethe individual beat cycles of the cells. If a particular gene isrequired for development of cardiomyocytes from ES cells, it is likelythat the knockout of that gene will either block or delay thedifferentiation of ES cells to cardiomyocytes. Since the impedance-basedmeasurement system is capable of functional monitoring of cardiomyocyte,it can be used as a specific way to monitor the effect of either geneknockout or transgene expression on cardiomyocyte function. Inalternative embodiments, the ES cells can be transfected with specificsiRNA to “knockdown” the product of a particular transcript and thenmonitor cardiomyocyte differentiation and function in vitro using theimpedance-based measurement system with millisecond time resolution.

In other exemplary embodiments an impedance-based measurement system isused to evaluate the role of genes in cardiac viability, morphology,development and or beating function. The steps involved in assessing thedevelopmental and functional effect of gene knockout or transgeneexpression may include: obtaining ES cells harboring specific knockoutof genes or which expresses a particular transgene; providing asingle-well or multi-well device that comprise microelectrode arrays inwell(s) of the device, which can be used for monitoring cell-substrateimpedance; optionally coat wells of the device with either fibronectinor other matrix proteins; seeding the embryonic stem cells (ES cells) ofmammalian origin or adult stem cells of mammalian origin at specificseeding densities to the wells of the device; allowing the cells toattach and spread and monitor the growth and viability of the cellsusing the impedance-based system; after a specified period of timeunique to ES-derived cardiomyocytes or primary cardiomyocytes,monitoring cardiomyocyte beating using the impedance-monitoring systemto monitor electrode impedance by using milli-second kinetic readout toresolve the individual beat cycles of the cells. If a particular gene isrequired for development of cardiomyocytes from ES cells, it is likelythat the knockout of that gene will either affect the viability of thecells or block or delay the differentiation of ES cells tocardiomyocytes. Since the impedance-based measurement system is capableof functional monitoring of cardiomyocyte both in long term assays andshort term assays, it can be used as a specific way to monitor theeffect of either gene knockout or transgene expression on cardiomyocytefunction. In related embodiments, the ES cells can be transfected withspecific siRNA to “knockdown” the product of a particular transcript andthen monitor cardiomyocyte viability, differentiation and function invitro using the impedance-based measurement system.

In another exemplary embodiment, steps involved in assessing thedevelopmental and functional effect of gene knockout or transgeneexpression using an impedance-based system in vitro are provided, whichinclude: obtaining ES cells harboring specific knockout of genes orwhich expresses a particular transgene; optionally coating E-PLATES witheither fibronectin or other matrix proteins; seeding either ES cells ofmammalian origin or primary cardiomyocytes at specific seedingdensities; allowing the cells to attach and spread; and after aspecified period of time unique to ES-derived cardiomyocytes or primarycardiomyocytes, monitoring cardiomyocyte beating using theimpedance-based system by using millisecond kinetic readout to resolvethe individual beat cycles of the cells. If a particular gene isrequired for development of cardiomyocytes from ES cells, it is likelythat the knockout of that gene will either block or delay thedifferentiation of ES cells to cardiomyocytes. Since the impedance-basedsystem is capable of functional monitoring of cardiomyocyte, it can beused as a specific way to monitor the effect of either gene knockout ortransgene expression on cardiomyocyte function. Alternatively, the EScells can be transfected with specific siRNA to “knockdown” the productof a particular transcript and then monitor cardiomyocytedifferentiation and function in vitro using the impedance-based system.

In another aspect, the present invention is directed to method toestablish an assay to assess the effect of gene knockout or transgeneexpression in ES cells differentiated to cardiomyocytes and functionallymonitored by the impedance-based system. The method includes providing adevice for measuring cell-substrate impedance operably connected to animpedance analyzer, wherein the device includes at least two wells;adding wildtype ES cells as control to at least 1 well and ES cells witha gene knockout or a transgene in at least 1 other well; monitoringimpedance of the at least two wells at time intervals over a period oftime and optionally determining cell indices from impedance values;comparing the impedance-based curves or optionally the cell index curvesbetween the control well and the well containing the ES cells harboringa knockout of a specific gene or expressing a specific transgene; and ifsignificantly different, concluding that the gene knockout or thetransgene can affect either cardiomyocyte viability, morphology from EScells or cardiomyocyte function as monitored by observing theexcitation-contraction coupling.

As above, in preferred embodiments, the time difference between twoconsecutive impedance measurement for a well is in the range ofmilliseconds (e.g., less than 500 milliseconds, less than 300milliseconds, less than 100 milliseconds, less than 10 milliseconds, orless than about 1 millisecond or faster). The millisecond kineticreadout is required to resolve the individual beat cycles of the cells.Thus, the time resolution for the impedance measurement should allow thesystem to perform measurement using at least two time points for eachbeat cycle. In some embodiments more than two time points are resolvedfor each beating cycle.

EXAMPLES Example 1 Impedance Monitoring of Attachment, Growth andViability of Mouse ES Cell-Derived Cardiomyocytes

Since isolated primary cardiomyocytes, ES-derived cardiomyocytes andadult stem cell-derived cardiomyocytes can be maintained in culture,these cells provide a desirable model system to study cardiomyocytes aswell as cardiomyocyte modulation. The present example demonstrates theuse of the ACEA RT-CES system to measure and monitor the attachment,growth and viability of mouse ES-derived cardiomyocytes which wereseeded at different seeding densities.

Mouse ES cells were seeded at a density ranging from 3000 cells to50,000 cells per well in E-PLATES (Acea Biosciences, San Diego, Calif.)precoated with fibronectin. The attachment, growth and viability of thecells were monitored on RT-CES system measuring impedance signal in theform of cell index every 30 minute for 48 hours. At about 48 hrs aftercell seeding, the growth of the cells had ceased and the appearance ofbeating cardiomyocytes were evident as judged by looking at the cellsinside the E-PLATE under the microscope.

FIG. 3 shows the cell index curves measured on RT-CES system for 4different seeding densities (3750, 7500, 15,000 and 30,000 cells perwell) of mouse ES-derived cardiomyocytes. For such long termmeasurement, cell electrode impedance and corresponding cell indiceswere measured at about 15 minute intervals. Based on the cell indexgrowth and viability curves, it is evident that the extent of theimpedance signal correlates well with the seeding density of viableES-derived cardiomyocytes.

Example 2 Detection of Cardiotoxic Effects Using Real Tim ImpedanceMonitoring

As an example, we describe here the use of the ACEA RT-CES system tomeasure and monitor the attachment and growth of mouse ES cells derivedcardiomyocytes and subsequently treated with a cytotoxic agent (FIG. 4).

Mouse ES-derived cardiomyocytes were seeded at a density of 25,000 cellsper well in E-PLATES precoated with fibronectin. The attachment andgrowth of cells were monitored on RT-CES system for 72 hours and thentreated with increasing doses of the compound of sodium dichromatedehydrate which is known to induce cytotoxicity. According to FIG. 4Athe plot of normalized cell index for cells in different wells treatedwith different concentrations of sodium dichromate dehydrate (SDD), SDDcauses a concentration dependent decrease in viability of ES-derivedcardiomyocytes. To quantify the extent of sodium dichromate dehydrateactivity against the cardiomyocytes, the normalized cell indices at 24hrs after compound treatment were plotted against the log of thecorresponding sodium dichromate dehydrate concentrations. From thesigmoidal curve, shown in FIG. 4B, half maximal activity or IC-50 valueof 1.48 uM was derived for the compound.

Example 3 Real Time Monitoring of Mouse ES Cell-Derived Cardiomyocytesand Inducing Hypertrophy

As an example, we describe here the use of the ACEA RT-CES system tomeasure and monitor the attachment and growth of mouse ES cells derivedcardiomyocytes and subsequently treated with isoproteranol, a β2adrenergic receptor agonist known to induce hypertrophy (FIG. 5A).

Mouse ES-derived cardiomyocytes were seeded at a density of 25,000 cellsper well in E-PLATES precoated with fibronectin. The attachment andgrowth of the cells were monitored on RT-CES system for 72 hours andthen treated with increasing doses of the compound isoproteranol.According to FIG. 5A, isoproteranol causes a concentration dependentchange in cell index readings. The timing of the cell index change isconsistent with a change in the morphology of the cells which we haveshown previously for other GPCR agonists in primary cells (Yu et al(2006): Real-time monitoring of morphological changes in living cells byelectronic cell sensor arrays: an approach to study G protein-coupledreceptors; Analytical Chemistry, Vol 78, pages 35-43). To quantify theextent of isoproteranol-induced morphological changes in mouseES-derived cardiomyocytes, the normalized cell indices were plottedagainst the log of the corresponding isoproteranol concentrations (FIG.5B). From the sigmoidal curve generated a half maximal activity or IC-50value of 3.1 nM was derived for the compound.

Example 4 Real Time Impedance Monitoring with Millisecond Resolution ofBeating Cardiomyocytes

As an example, we describe here the use of an improved impedance-basedsystem with millisecond time resolution to measure and monitor thebeating of cardiomocytes using fast kinetic software.

Mouse ES cells were seeded at a density of between 3,000 to 50,000 cellsper well in E-PLATES precoated with fibronectin. The attachment andgrowth of the cells were monitored on RT-CES system. FIG. 6 shows thecell index curves measured on RT-CES system for 4 individual wells, withcell culture medium as background starting from cell seeding to about 86hrs after cell seeding. For such long term measurement, cell electrodeimpedance and corresponding cell indices were measured at about 15minute intervals. As evidenced on these plots, the cell index curveswere rather smooth up to about 44-48 hrs, after which there were“noises” or “small-spikes” on the cell index curves. Such spikes weremost evident after about 60 hrs. We now understand the origins of such“small-spikes” and interpret that such spikes in the impedance or cellindex readout are associated with the beating of the cells. During thesynchronized beating of the cells, the cell morphology and celladhesion/attachment to the electrodes change regularly in synchrony withthe cell beating. Such regular or periodic changes in cell morphologyand cell adhesion/attachment are then reflected in the changes incell-electrode or cell-substrate impedances.

In order to monitor beating of cardiomyocytes, a specially designedsoftware and measurement circuit hardware that are capable ofmillisecond impedance data acquisition (e.g., typically with timeresolution between consecutive impedance measurement for a same wellbeing less than 40 milliseconds) was used to monitor the quick rhythmicbeating of the cardiomyocytes. For such measurement, the backgroundelectrode impedance is measured with the cells inside the wells (note,this is in contrast with FIGS. 3 and 6, where the background measurementis performed using cell culture media). The software was used to measurecardiomyocyte beating at distinct stages throughout its attachment andgrowth phases (FIG. 7). For a baseline reference, the impedancemeasurement was done on the ES cells at 24 hours where the cells had notfully spread and formed a tight monolayer and even though the cellsappear to beat when visualized under a microscope, they do soasynchronously and as a result no net beating signal is detected (FIG.7A). For plot, impedance readout has been converted into dimensionlesscell indices. FIGS. 7B and 7C shows that ES cells that had fully spreadand formed tight junctions with neighboring cells at 48 hours and 72hours respectively, show regular impedance-spikes which correlate withthe beating frequency of cardiomyocytes as judged by microscopicobservation.

To use the measured cell index curves, it is important to further derivevarious physiologically relevant parameters. Several importantparameters may include, the beating rate of the cardiomyocytes (i.e.,how many times the cells beat within a unit of time for example, aminute), the beating amplitude (i.e. the magnitude of the beating ofcells in terms of impedance change) the average amplitude intensity in aunit time as well as the average length of time between the beats, timeof rise for a beat, time of decay for a beat. Because of the unique andcomplex nature of the impedance readout signals (smaller signalamplitude, sampling time resolution may be limited by the hardware andthe software used), appropriate methods or techniques are required foranalyzing cell index curves to derive the above mentioned parameters.

For deriving the beating rates of cardiomyocytes, one method may be bycounting how many peaks there are within a given time frame (forexample, one minute). For this approach to work, the sampling timeresolution has to be sufficiently high so that the each beat of thecells does show a peak on the recorded cell index curves. In addition,determining a peak “automatically” also require some algorithm. Forexample, each peak would have to have one “rise” in cell index and alsoone “decay” in cell decay. Each “rise”-and-“decay” pair forms a singlepeak. The algorithm needs to determine such “rise” and “decay” portionsof the curves and then counts a peak.

Another method to derive or count the beating rates of cardiomyocytes isto perform a detailed signal analysis to derive the frequency componentsof the cell index curves and to derive the magnitude of each frequencycomponents. One method of signal analysis is Fourier transform of thecell index curve (of the time domain). Like above method, the samplingtime-resolution needs to be sufficiently high so that each beat of thecells has at least three time points being measured. After performingFourier transform, we would look for the frequency components having thelargest magnitude and such frequency would correspond to, or be veryclose to, the beating frequency. In addition, for such analysis, givinga fixed sampling time resolution, the more time points sampled foranalysis, the more accurate it is for the analyzed beating frequency.FIG. 8A and FIG. 8B shows a pair of cell index curves and thecorresponding beat rate based the above described Fourier transform. InFIG. 8B, the cell index curves last from time zero to time 42 seconds.In FIG. 8B, the beating frequency for the traces in FIG. 8A is analyzedusing the method described here, i.e., Fourier transform followed bypicking up the highest-magnitude frequency component. For each derivedfrequency data at one time moment in FIG. 8B, cell index data frommultiple time points (starting from previous 98 time points plus thetime moment of interest) is used for analysis. Thus, the time axis inFIG. 8B starts from about 20 seconds to 42 seconds.

Another method deriving the beating rates of cardiomyocytes is to firstdetermine the time length (AT in seconds) between two consecutive peaksand then calculate the beating rate according to the formula of “beatsper minute=60/ΔT”. Thus for each two-consecutive peaks, one cancalculate one beating rate. Furthermore, one can plot this beating rateas a function of the time (of the first of the two consecutive peaks) toobtain the time dependency of the beating rates. FIG. 9A shows anexample of the time-dependent beat rates derived using this method, formouse ES-derived cardiomycoytes treated with compound sotalol at aconcentration of 4.4 uM. Corresponding cell index data is shown in FIGS.9C and 9D, where the time resolution between two adjacent points is 40milliseconds. In other words, a second in FIGS. 9C and 9D is equivalentto 40-milliseconds.

For deriving the amplitude of the beating of the cardiomyocytes, theremay also be different methods. One method is to analyze each peak andfinding the peak maximum and the peak minimum. The amplitude iscalculated by subtracting peak maximum by the peak minimum. Then, onecan plot the peak amplitude as a function of the time of the peak toobtain the time dependency of the peak amplitude. FIG. 9B shows anexample of the time-dependent peak amplitudes derived using this method,for mouse ES-derived cardiomycoytes treated with compound sotalol at aconcentration of 4.4 uM. Another approach may also be to use Fouriertransform described above. Then based on derived Fourier coefficients,one can re-simulate time domain cell index curves and look for the peakmagnitude from the simulated curves.

For deriving the averaged length of time between the beats, there mayalso be different methods. For each identified peak, one can firstdetermine a starting point of the peak. Then the time difference betweentwo consecutive peaks at the two starting points of the peak can be usedfor the length of time between the beats.

With the method of determining each peak, one can also calculate thetime-of-rise of the peak and the time-decay-of the peak.

Example 5 Identifying Modulators of Cardiac Function Using Real TimeImpedance Monitoring with Millisecond Resolution

Similar to Example 4, the millisecond kinetic readout requires that theimpedance measurement system can provide impedance measurement data atmillisecond time resolution. In other words, the time difference betweentwo consecutive impedance measurement for a well shall be in the rangeof milliseconds (e.g., less than 500 millisecond, less than 300millisecond, less than 100 millisecond, less than 10 millisecond, orless than 1 millisecond or faster). The millisecond kinetic readout isrequired to resolve the individual beat cycles of the cells. Thus, thetime resolution for the impedance measurement should allow the system toperform measurement at least two time points for each beat cycle, or atmore than two points for each beat cycle.

In order to demonstrate the utility of the millisecond kineticimpedance-measurements, we first used two pharmacological agents, oneknown to suppress the heart rate and consequently cardiomyocyte beatingand the other known to increase the heart rate and consequently the rateof cardiomyocyte beating. As Mouse ES cells were seeded in FN-coatedE-PLATES and monitored for about 72 hours when the cells differentiatedinto beating cardiomyocytes, as described in Section 4. A baseline ofthe cardiomyocyte was taken for approximately 40 seconds using thespecially designed fast kinetic data acquisition hardware and softwarewhich is capable of millisecond data acquisition and display. An agonistof muscarinic receptors, carbachol, which is known to slow down theheart rate was added to one well at a final concentration of 333 nM andcardiomyocyte beating was monitored for 10 minutes (FIG. 10A). The dataclearly shows that carbachol significantly slows down the rate ofcardiomyocyte beating from 80 beats/min prior to carbachol addition to˜60 beats/min after carbachol addition (Table I). Alternatively,addition of isoproteranol at a final concentration of 4.4 uMsignificantly increased the rate of cardiomyocyte beating from ˜65beats/min to 115-135 beats/min (FIG. 10B and Table I). These dataclearly show that the readout system and the fast kinetic software aresufficiently robust and sensitive to detect these changes in rate ofcardiomyocyte beating even at very low compound concentrations. Similarto the cell index plot shown in FIGS. 9C and 9D, the time resolutionbetween two adjacent points in FIGS. 10A and 10B is 40 milliseconds. Inother words, a second in FIGS. 10A and 10B is equivalent to 40millisecond.

TABLE I Beat Compound Mechanism Concentration Rate Amplitude PatternChange Carbachol Muscarnic receptor  333 nM From ~80 From 0.07 Beatingrate agonist to ~60 to 0.06 decreased Isoproteranol b2 Adrenergic  4.4uM From ~65 From 0.19 Beating rate receptor agonist to 115-136 to 0.16increased

To further demonstrate the capabilities of the improved impedance-basedmonitoring of cardiomyocyte beating in detecting drugs which mayadversely affect heart function, a number of drugs which have beenpulled out of the market due to cardiotoxic side effects such as ERGchannel inhibition and QT elongation were compiled and tested in adose-dependent manner. The list of these compounds, their mechanism andadverse side affects are shown in Table II. For these tests, mouse ESderived cardiomyocytes were seeded at a final density of 25,000 cells inACEA E-Plates and continually monitored by the RT-CES system.Approximately, 72 hours after cell seeding the improved,impedance-measurement system with millisecond time resolution was usedto establish a baseline reading of cardiomyocyte beating for each wellfor about 40 seconds. Subsequently, the cells in each well were treatedwith the indicated drug and dose shown in FIG. 11. Similar to the cellindex plot shown in FIGS. 9C and 9D, the time resolution between twoadjacent points in all the figures in FIG. 9 is 40 milliseconds. Inother words, a second in FIG. 9 is equivalent to 40-milliseconds.

Table II shows a summary of the results, clearly demonstrating thatcompounds which have been shown to affect ERG channels do affect variousaspects of cardiomyocyte beating and function such as frequency ofbeating, magnitude of beating. Furthermore, as shown in FIG. 12 some ofthese compounds can lead to qualitatively different or similar patterns.For example, the compounds E4031, Astezimole and dofetilide which areERG channel inhibitors do contain patterns with similar features. Insummary these results clearly indicate that using theimpedance-measurement system with millisecond time resolution do lead toa sensitive and robust readout for cardiomyocyte beating that can alsodetect drugs which are known to be cardiotoxic. Similar to the cellindex plot shown in FIGS. 9C and 9D, the time resolution between twoadjacent points in all the figures in FIG. 10 is 40 milliseconds. Inother words, a second in a figure in FIG. 10 is equivalent to40-millisecond.

TABLE II Compound Mechanism Concentration Beat Rate Amplitude PatternChange Astemizole anti-histamine 400 nM 0 <0.01 From 68-72 bpm toirregular being to beating stopped Terfenadine anti-histamine 200 nM 0<0.01 From 71-78 bpm to irregular beating to beating stoppedErythromycin anti-biotic 13.3 uM From ~60 to ~80 0.09 to 0.07 No patternchange Moxifloxacin anti-biotic 20 uM From ~80 to ~73 From 0.055 to0.060 No pattern change Pentamidine anti-infective 20 uM From 71~78 to~71 From 0.24 to 0.21 No pattern change Amitriptyline Serotonergic 4.4uM From ~70 to ~90 From 0.22 to 0.17 No pattern change InhibitorVerpamil Ca channel 130 uM From ~65 to 0 From 0.13 to 0.06 From ~65 bpmto only blocker occasional single beating. Rosglitazone PPAR agonist13.3 uM From ~79 to ~75 From 0.22 to 0.17 No pattern change Dofitlite500 uM From ~80 to ~180 From 0.09 to 0.02 Pattern changed, much fasterRofecoxib COX-2 Inhibitor 13.3 uM From ~68 to ~60 From 0.2 to 0.19 Nopattern change Rofecoxib COX-2 Inhibitor 40 uM From ~62 to ~60 From 0.16to 0.10 Pattern change, no beating after initial treatment, thenrecovers Celecoxib COX-2 Inhibitor 4.4 uM From ~60 to (~20-~50) From~0.2 to ~0.12 Pattern change Doxirubicin Anthracycline 40 uM ~70 ~0.16No pattern change (initially). Beating pattern changes after 2 hrs.Cyclosporin A Calcineurin 13.3 uM From ~70 to ~80 From 0.18 to 0.15 Nopattern change inhibitor Propalanol β2-adrenergic 4.4 uM From ~70 toover 150 From 0.25 to 0.025 Pattern changed, much receptor faster andirregular antagonist Sotalol 13.3 uM From ~80 to ~160 From 0.27 to 0.07Much faster, pattern changed E4031 K channel 120 nM From 80 to 160 From0.09 to 0.03 Pattern changed, much inhibitor faster DDT Pesticide 8 uMFrom 80 to 140, From 0.25 to 0.02 Inititally become faster, then to ~80later irregular beating PCB Organic toxicant 8 uM From ~65 to o From 0.2to 0 Beating stopped Endosulfan insecticide 8 uM From ~75 to o From 0.2to 0 Beating stopped

To further demonstrate the capabilities of the fastimpedance-measurement-system-based monitoring of cardiomyocyte beatingin detecting drugs which may adversely affect heart function, compoundsat different concentrations were tested to demonstrate thedose-dependent effects of these compounds on cardiomyocytes.Approximately, 72 hours after cell seeding the impedance-measurementsystem with millisecond time resolution was used to establish a baselinereading of cardiomyocyte beating for each well for about 40 seconds.Subsequently, the cells in each well were treated with drugs atdifferent dose concentrations. FIG. 13 shows an example of dosedependent effects of Astemizole on cardiomyocytes beating at differentconcentrations. At high concentration of 400 nM, Astemizole had such astrong effect on the beating of cardiomyocytes that the beating almoststopped. The effect of Astemizole on the beating of the cardiomyocytesis clearly does-dependent. At low concentration of 40 pM, its effect onthe beating of the cardiomyocytes is small that the cardiomyocytebeating rate was not affected. Similar to the cell index plot shown inFIGS. 9C and 9D, the time resolution between two adjacent points in allthe figures in FIG. 11 is 40 milliseconds. In other words, a second inFIG. 13 is equivalent to 40 milliseconds.

Example 6 Modulation of Cardiomyocyte Beating in Response to HaloperidolTreatment Using Millisecond Time Resolution

To further demonstrate the capabilities of the improved impedancemeasurement system for monitoring of cardiomyocyte beating in detectingdrugs which may adversely affect heart function, 500 nM Haloperidol wastested to demonstrate its time-dependent effects on cardiomyocytes.Approximately, 72 hours after cell seeding the improved impedance systemwas used to establish a baseline reading of cardiomyocyte beating foreach well for about 16 seconds (FIG. 14A). Note that in FIG. 14A, eachminute showed on the x-axis of FIG. 14A corresponds to one second inreal measurement. Subsequently, the cells in the well treated with 500nM Haloperidol. FIGS. 14B, C, D, E, F, G and H show the impedance-basedmonitoring of beating of the cardiomyocytes at time points of first 30seconds, about 1 minute, about 2 minutes, about 4 minutes, about 5.5minutes, about 1 hour and about 21 hours after the compound treatment.In FIGS. 14B through 14H, each minute showed on the x-axis correspondsto one second in real measurement. It is clear that within 1 hr ofcompound treatment, the cardiomyocytes exhibit significant,time-dependent change in their impedance responses. Even at ˜21 hrsafter compound treatment, the impedance pattern did not restore and thebeating frequency (50˜60 beats per minute) appeared to become about halfof that before compound treatment (90˜100 beats per minute beforetreatment). Yet the amplitude of the impedance response spikes nearlydoubled compared with that before compound treatment. FIG. 15 and FIG.16 show the analysis results of the impedance beating pattern of FIG. 14in terms of the beating frequency (beats per minute) and the beatingmagnitude For FIG. 15 and FIG. 16, each unit (1) on the x-axiscorresponds to 0.016 second (16 milliseconds) in real measurement.

Example 7 Modulation of Cardiomyocyte Beating in Response to EryromycinTreatment Using Millisecond Time Resolution

To further demonstrate the capabilities of the improved impedancemeasurement system for monitoring of cardiomyocyte beating in detectingdrugs which may adversely affect heart function, 300 uM erythromycin wastested to demonstrate its time-dependent effects on cardiomyocytes.Approximately, 72 hours after cell seeding the improved impedance systemwas used to establish a baseline reading of cardiomyocyte beating foreach well for about 15 seconds (FIG. 17A). Note that in FIG. 17A, eachminute showed on the x-axis of FIG. 17A corresponds to one second inreal measurement. Subsequently, the cells in the well treated with 300uM erythromycin. FIGS. 17B-I show the impedance-based monitoring ofbeating of the cardiomyocytes at time points of first 30 seconds, about1 minute, about 2 minutes, about 4 minutes, about 5.5 minutes, about 1hour, about 3 hr and about 21 hours after the compound treatment. InFIGS. 17B-H, each minute showed on the x-axis corresponds to one secondin real measurement. It is clear that within 3 hr of compound treatment,the cardiomyocytes exhibit significant, time-dependent change in theirimpedance responses. For example, at about 1 hr after the treatment, thefrequency of the impedance-based beating spikes is much higher than thatfor the cardiomyocytes before treatment, yet the magnitude is muchsmaller. On the other hand, at about 3 hr after treatment, theimpedance-based beating pattern becomes rather irregular whilst themagnitude of the beating pattern has somewhat restored. Still, even at˜21 hrs after compound treatment, the impedance pattern did not restoreand the beating frequency appeared to about 70% (at 50˜60 beats perminute) of that before compound treatment (at 80˜90 beats per minute).Yet the amplitude of the impedance response spikes increased by about50% compared with that before compound treatment. FIG. 18 and FIG. 19show the analysis results of the impedance beating pattern of FIG. 17 interms of the beating frequency (beats per minute) and the beatingmagnitude. For FIG. 18 and FIG. 19, each unit (1) on the x-axiscorresponds to 0.016 second (16 milli-seconds) in real measurement.

What is claimed is:
 1. A system for monitoring impedance of excitablecells in vitro, comprising: a) a device for monitoring cell-substrateimpedance at 20 millisecond resolution, the device comprising i) anonconductive substrate; ii) one or more electrode arrays fabricated onsaid substrate; iii) one or more wells on said substrate, wherein eachof said one or more arrays is associated with one of said one or morewells; wherein each electrode array is individually addressable, furtherwherein said substrate has a surface suitable for cell attachment,wherein said cell attachment on said substrate can result in adetectable change in impedance between electrodes within each electrodearray; b) an impedance analyzer capable of impedance measurement at 20millisecond time resolution; c) electronic circuitry that can engagesaid device and selectively connect said two or more electrode arrays ofsaid device to said impedance analyzer; and d) a software program thatcontrols said electronic circuitry and records and analyzes dataobtained from said impedance analyzer.
 2. The system according to claim1, wherein each of said electrode arrays comprises two electrodestructures having substantially a same surface area.
 3. The systemaccording to claim 2, wherein said electrode structures are eachconstructed from a material selected from the group consisting of metal,gold, platinum, titanium and chromium.
 4. The system according to claim2, wherein said electrode structures have a width between 20 microns and500 microns.
 5. The system according to claim 2, wherein a gap betweenelectrode structures within each electrode array is between 5 micronsand 100 microns.
 6. The system according to claim 1, wherein eachelectrode array is organized in a configuration selected from the groupconsisting of interdigitated, circle-on-line, diamond-on-line,concentric, sinusoidal, and castellated.
 7. The system according toclaim 1, wherein said 20 millisecond time resolution allows forimpedance measurements at more than two time points within eachexcitation cycle of excitable cells introduced to wells.
 8. The systemaccording to claim 7, wherein said excitation cycle is a cell beatingcycle.
 9. The system according to claim 8, wherein said excitable cellsare cardiomyocytes.
 10. The system according to claim 7, wherein saidexcitable cells undergo excitation-contraction coupling.
 11. The systemaccording to claim 1, wherein said impedance measurement at 20millisecond time resolution comprises at least two consecutive impedancemeasurements 20 milliseconds or less apart.
 12. The system according toclaim 11, wherein said at least two consecutive impedance measurementsare less than 10 milliseconds apart.
 13. The system according to claim1, wherein said one or more wells is selected from the group consistingof 6 wells, 8 wells, 16 wells, 32 wells, 96 wells and 384 wells.